diff --git a/CHANGES b/CHANGES
index ec7707a5ef0..ab49ff15503 100644
--- a/CHANGES
+++ b/CHANGES
@@ -21,12 +21,18 @@ Incompatible changes
   intersphinx should resolve, or explicitly set the value of this configuration
   variable to an empty list.
 * #9999: LaTeX: separate terms from their definitions by a CR (refs: #9985)
+* #10062: Change the default language to ``'en'`` if any language is not set in
+  ``conf.py``
 
 Deprecated
 ----------
 
 * setuptools integration.  The ``build_sphinx`` sub-command for setup.py is
   marked as deprecated to follow the policy of setuptools team.
+* The ``locale`` argument of ``sphinx.util.i18n:babel_format_date()`` becomes
+  required
+* The ``language`` argument of ``sphinx.util.i18n:format_date()`` becomes
+  required
 * ``sphinx.writers.latex.LaTeXWriter.docclasses``
 
 Features added
diff --git a/doc/extdev/deprecated.rst b/doc/extdev/deprecated.rst
index 159c6d8b68a..1d3ad693087 100644
--- a/doc/extdev/deprecated.rst
+++ b/doc/extdev/deprecated.rst
@@ -27,6 +27,16 @@ The following is a list of deprecated interfaces.
      - 7.0
      - N/A
 
+   * - The ``locale`` argument of ``sphinx.util.i18n:babel_format_date()``
+     - 5.0
+     - 7.0
+     - N/A
+
+   * - The ``language`` argument of ``sphinx.util.i18n:format_date()``
+     - 5.0
+     - 7.0
+     - N/A
+
    * - ``sphinx.writers.latex.LaTeXWriter.docclasses``
      - 5.0
      - 7.0
diff --git a/sphinx/application.py b/sphinx/application.py
index fd8b65b6ef6..103ebb6340c 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -266,7 +266,7 @@ def _init_i18n(self) -> None:
         """Load translated strings from the configured localedirs if enabled in
         the configuration.
         """
-        if self.config.language is None:
+        if self.config.language == 'en':
             self.translator, has_translation = locale.init([], None)
         else:
             logger.info(bold(__('loading translations [%s]... ') % self.config.language),
@@ -285,8 +285,7 @@ def _init_i18n(self) -> None:
             locale_dirs += [path.join(package_dir, 'locale')]
 
             self.translator, has_translation = locale.init(locale_dirs, self.config.language)
-            if has_translation or self.config.language == 'en':
-                # "en" never needs to be translated
+            if has_translation:
                 logger.info(__('done'))
             else:
                 logger.info(__('not available for built-in messages'))
diff --git a/sphinx/builders/html/__init__.py b/sphinx/builders/html/__init__.py
index ac1fbbaec9d..674b5147f0e 100644
--- a/sphinx/builders/html/__init__.py
+++ b/sphinx/builders/html/__init__.py
@@ -326,7 +326,7 @@ def init_js_files(self) -> None:
             attrs.setdefault('priority', 800)  # User's JSs are loaded after extensions'
             self.add_js_file(filename, **attrs)
 
-        if self.config.language and self._get_translations_js():
+        if self._get_translations_js():
             self.add_js_file('translations.js')
 
     def add_js_file(self, filename: str, **kwargs: Any) -> None:
@@ -431,8 +431,6 @@ def prepare_writing(self, docnames: Set[str]) -> None:
         if self.search:
             from sphinx.search import IndexBuilder
             lang = self.config.html_search_language or self.config.language
-            if not lang:
-                lang = 'en'
             self.indexer = IndexBuilder(self.env, lang,
                                         self.config.html_search_options,
                                         self.config.html_search_scorer)
@@ -767,10 +765,9 @@ def create_pygments_style_file(self) -> None:
 
     def copy_translation_js(self) -> None:
         """Copy a JavaScript file for translations."""
-        if self.config.language is not None:
-            jsfile = self._get_translations_js()
-            if jsfile:
-                copyfile(jsfile, path.join(self.outdir, '_static', 'translations.js'))
+        jsfile = self._get_translations_js()
+        if jsfile:
+            copyfile(jsfile, path.join(self.outdir, '_static', 'translations.js'))
 
     def copy_stemmer_js(self) -> None:
         """Copy a JavaScript file for stemmer."""
diff --git a/sphinx/builders/latex/__init__.py b/sphinx/builders/latex/__init__.py
index 139973c3241..ccb92f47e6a 100644
--- a/sphinx/builders/latex/__init__.py
+++ b/sphinx/builders/latex/__init__.py
@@ -170,9 +170,8 @@ def init_context(self) -> None:
         self.context.update(ADDITIONAL_SETTINGS.get(self.config.latex_engine, {}))
 
         # Add special settings for (latex_engine, language_code)
-        if self.config.language:
-            key = (self.config.latex_engine, self.config.language[:2])
-            self.context.update(ADDITIONAL_SETTINGS.get(key, {}))
+        key = (self.config.latex_engine, self.config.language[:2])
+        self.context.update(ADDITIONAL_SETTINGS.get(key, {}))
 
         # Apply user settings to context
         self.context.update(self.config.latex_elements)
@@ -203,7 +202,7 @@ def update_context(self) -> None:
 
     def init_babel(self) -> None:
         self.babel = ExtBabel(self.config.language, not self.context['babel'])
-        if self.config.language and not self.babel.is_supported_language():
+        if not self.babel.is_supported_language():
             # emit warning if specified language is invalid
             # (only emitting, nothing changed to processing)
             logger.warning(__('no Babel option known for language %r'),
@@ -232,12 +231,11 @@ def init_multilingual(self) -> None:
             self.context['classoptions'] += ',' + self.babel.get_language()
             # this branch is not taken for xelatex/lualatex if default settings
             self.context['multilingual'] = self.context['babel']
-            if self.config.language:
-                self.context['shorthandoff'] = SHORTHANDOFF
+            self.context['shorthandoff'] = SHORTHANDOFF
 
-                # Times fonts don't work with Cyrillic languages
-                if self.babel.uses_cyrillic() and 'fontpkg' not in self.config.latex_elements:
-                    self.context['fontpkg'] = ''
+            # Times fonts don't work with Cyrillic languages
+            if self.babel.uses_cyrillic() and 'fontpkg' not in self.config.latex_elements:
+                self.context['fontpkg'] = ''
         elif self.context['polyglossia']:
             self.context['classoptions'] += ',' + self.babel.get_language()
             options = self.babel.get_mainlanguage_options()
@@ -380,14 +378,10 @@ def copy_support_files(self) -> None:
         # configure usage of xindy (impacts Makefile and latexmkrc)
         # FIXME: convert this rather to a confval with suitable default
         #        according to language ? but would require extra documentation
-        if self.config.language:
-            xindy_lang_option = \
-                XINDY_LANG_OPTIONS.get(self.config.language[:2],
-                                       '-L general -C utf8 ')
-            xindy_cyrillic = self.config.language[:2] in XINDY_CYRILLIC_SCRIPTS
-        else:
-            xindy_lang_option = '-L english -C utf8 '
-            xindy_cyrillic = False
+        xindy_lang_option = XINDY_LANG_OPTIONS.get(self.config.language[:2],
+                                                   '-L general -C utf8 ')
+        xindy_cyrillic = self.config.language[:2] in XINDY_CYRILLIC_SCRIPTS
+
         context = {
             'latex_engine':      self.config.latex_engine,
             'xindy_use':         self.config.latex_use_xindy,
@@ -474,7 +468,7 @@ def default_latex_engine(config: Config) -> str:
     """ Better default latex_engine settings for specific languages. """
     if config.language == 'ja':
         return 'uplatex'
-    elif (config.language or '').startswith('zh'):
+    elif config.language.startswith('zh'):
         return 'xelatex'
     elif config.language == 'el':
         return 'xelatex'
diff --git a/sphinx/builders/latex/util.py b/sphinx/builders/latex/util.py
index 6b797c9a248..4a5c390c3bf 100644
--- a/sphinx/builders/latex/util.py
+++ b/sphinx/builders/latex/util.py
@@ -20,7 +20,7 @@ def __init__(self, language_code: str, use_polyglossia: bool = False) -> None:
         self.language_code = language_code
         self.use_polyglossia = use_polyglossia
         self.supported = True
-        super().__init__(language_code or '')
+        super().__init__(language_code)
 
     def uses_cyrillic(self) -> bool:
         return self.language in self.cyrillic_languages
diff --git a/sphinx/config.py b/sphinx/config.py
index 38ed1d38882..6ee036ebb30 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -100,7 +100,7 @@ class Config:
         # the real default is locale-dependent
         'today_fmt': (None, 'env', [str]),
 
-        'language': (None, 'env', [str]),
+        'language': ('en', 'env', [str]),
         'locale_dirs': (['locales'], 'env', []),
         'figure_language_filename': ('{root}.{language}{ext}', 'env', [str]),
         'gettext_allow_fuzzy_translations': (False, 'gettext', []),
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 9dde7b407d8..fa1b6e3689d 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -261,7 +261,7 @@ def _update_settings(self, config: Config) -> None:
         """Update settings by new config."""
         self.settings['input_encoding'] = config.source_encoding
         self.settings['trim_footnote_reference_space'] = config.trim_footnote_reference_space
-        self.settings['language_code'] = config.language or 'en'
+        self.settings['language_code'] = config.language
 
         # Allow to disable by 3rd party extension (workaround)
         self.settings.setdefault('smart_quotes', True)
diff --git a/sphinx/environment/collectors/asset.py b/sphinx/environment/collectors/asset.py
index 9530b3bd94d..74a183b8782 100644
--- a/sphinx/environment/collectors/asset.py
+++ b/sphinx/environment/collectors/asset.py
@@ -64,18 +64,16 @@ def process_doc(self, app: Sphinx, doctree: nodes.document) -> None:
                 rel_imgpath, full_imgpath = app.env.relfn2path(imguri, docname)
                 node['uri'] = rel_imgpath
 
-                if app.config.language:
-                    # Search language-specific figures at first
-                    i18n_imguri = get_image_filename_for_language(imguri, app.env)
-                    _, full_i18n_imgpath = app.env.relfn2path(i18n_imguri, docname)
-                    self.collect_candidates(app.env, full_i18n_imgpath, candidates, node)
+                # Search language-specific figures at first
+                i18n_imguri = get_image_filename_for_language(imguri, app.env)
+                _, full_i18n_imgpath = app.env.relfn2path(i18n_imguri, docname)
+                self.collect_candidates(app.env, full_i18n_imgpath, candidates, node)
 
                 self.collect_candidates(app.env, full_imgpath, candidates, node)
             else:
-                if app.config.language:
-                    # substitute imguri by figure_language_filename
-                    # (ex. foo.png -> foo.en.png)
-                    imguri = search_image_for_language(imguri, app.env)
+                # substitute imguri by figure_language_filename
+                # (ex. foo.png -> foo.en.png)
+                imguri = search_image_for_language(imguri, app.env)
 
                 # Update `node['uri']` to a relative path from srcdir
                 # from a relative path from current document.
diff --git a/sphinx/util/i18n.py b/sphinx/util/i18n.py
index 511a9abee97..db94786c239 100644
--- a/sphinx/util/i18n.py
+++ b/sphinx/util/i18n.py
@@ -10,14 +10,16 @@
 
 import os
 import re
+import warnings
 from datetime import datetime, timezone
 from os import path
-from typing import TYPE_CHECKING, Callable, Generator, List, NamedTuple, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Callable, Generator, List, NamedTuple, Tuple, Union
 
 import babel.dates
 from babel.messages.mofile import write_mo
 from babel.messages.pofile import read_po
 
+from sphinx.deprecation import RemovedInSphinx70Warning
 from sphinx.errors import SphinxError
 from sphinx.locale import __
 from sphinx.util import logging
@@ -173,9 +175,11 @@ def docname_to_domain(docname: str, compaction: Union[bool, str]) -> str:
 date_format_re = re.compile('(%s)' % '|'.join(date_format_mappings))
 
 
-def babel_format_date(date: datetime, format: str, locale: Optional[str],
+def babel_format_date(date: datetime, format: str, locale: str,
                       formatter: Callable = babel.dates.format_date) -> str:
     if locale is None:
+        warnings.warn('The locale argument for babel_format_date() becomes required.',
+                      RemovedInSphinx70Warning)
         locale = 'en'
 
     # Check if we have the tzinfo attribute. If not we cannot do any time
@@ -194,7 +198,7 @@ def babel_format_date(date: datetime, format: str, locale: Optional[str],
         return format
 
 
-def format_date(format: str, date: datetime = None, language: Optional[str] = None) -> str:
+def format_date(format: str, date: datetime = None, language: str = None) -> str:
     if date is None:
         # If time is not specified, try to use $SOURCE_DATE_EPOCH variable
         # See https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal
@@ -204,6 +208,11 @@ def format_date(format: str, date: datetime = None, language: Optional[str] = No
         else:
             date = datetime.now(timezone.utc).astimezone()
 
+    if language is None:
+        warnings.warn('The language argument for format_date() becomes required.',
+                      RemovedInSphinx70Warning)
+        language = 'en'
+
     result = []
     tokens = date_format_re.split(format)
     for token in tokens:
@@ -229,9 +238,6 @@ def format_date(format: str, date: datetime = None, language: Optional[str] = No
 
 
 def get_image_filename_for_language(filename: str, env: "BuildEnvironment") -> str:
-    if not env.config.language:
-        return filename
-
     filename_format = env.config.figure_language_filename
     d = dict()
     d['root'], d['ext'] = path.splitext(filename)
@@ -252,9 +258,6 @@ def get_image_filename_for_language(filename: str, env: "BuildEnvironment") -> s
 
 
 def search_image_for_language(filename: str, env: "BuildEnvironment") -> str:
-    if not env.config.language:
-        return filename
-
     translated = get_image_filename_for_language(filename, env)
     _, abspath = env.relfn2path(translated)
     if path.exists(abspath):
diff --git a/sphinx/writers/latex.py b/sphinx/writers/latex.py
index 1063392dd42..3037c8fa4a8 100644
--- a/sphinx/writers/latex.py
+++ b/sphinx/writers/latex.py
@@ -333,7 +333,7 @@ def __init__(self, document: nodes.document, builder: "LaTeXBuilder",
         if self.config.numfig and self.config.math_numfig:
             sphinxpkgoptions.append('mathnumfig')
 
-        if (self.config.language not in {None, 'en', 'ja'} and
+        if (self.config.language not in {'en', 'ja'} and
                 'fncychap' not in self.config.latex_elements):
             # use Sonny style if any language specified (except English)
             self.elements['fncychap'] = (r'\usepackage[Sonny]{fncychap}' + CR +
@@ -341,7 +341,7 @@ def __init__(self, document: nodes.document, builder: "LaTeXBuilder",
                                          r'\ChTitleVar{\Large\normalfont\sffamily}')
 
         self.babel = self.builder.babel
-        if self.config.language and not self.babel.is_supported_language():
+        if not self.babel.is_supported_language():
             # emit warning if specified language is invalid
             # (only emitting, nothing changed to processing)
             logger.warning(__('no Babel option known for language %r'),
diff --git a/tests/test_build_latex.py b/tests/test_build_latex.py
index 2c686cda731..1c3a8afeb10 100644
--- a/tests/test_build_latex.py
+++ b/tests/test_build_latex.py
@@ -528,7 +528,7 @@ def test_babel_with_no_language_settings(app, status, warning):
     assert '\\usepackage[Bjarne]{fncychap}' in result
     assert ('\\addto\\captionsenglish{\\renewcommand{\\contentsname}{Table of content}}\n'
             in result)
-    assert '\\shorthandoff' not in result
+    assert '\\shorthandoff{"}' in result
 
     # sphinxmessages.sty
     result = (app.outdir / 'sphinxmessages.sty').read_text()
diff --git a/tests/test_util_i18n.py b/tests/test_util_i18n.py
index 3f4e8d52a80..49a7067f534 100644
--- a/tests/test_util_i18n.py
+++ b/tests/test_util_i18n.py
@@ -98,15 +98,6 @@ def test_format_date():
 def test_get_filename_for_language(app):
     app.env.temp_data['docname'] = 'index'
 
-    # language is None
-    app.env.config.language = None
-    assert app.env.config.language is None
-    assert i18n.get_image_filename_for_language('foo.png', app.env) == 'foo.png'
-    assert i18n.get_image_filename_for_language('foo.bar.png', app.env) == 'foo.bar.png'
-    assert i18n.get_image_filename_for_language('subdir/foo.png', app.env) == 'subdir/foo.png'
-    assert i18n.get_image_filename_for_language('../foo.png', app.env) == '../foo.png'
-    assert i18n.get_image_filename_for_language('foo', app.env) == 'foo'
-
     # language is en
     app.env.config.language = 'en'
     assert i18n.get_image_filename_for_language('foo.png', app.env) == 'foo.en.png'
@@ -115,15 +106,6 @@ def test_get_filename_for_language(app):
     assert i18n.get_image_filename_for_language('../foo.png', app.env) == '../foo.en.png'
     assert i18n.get_image_filename_for_language('foo', app.env) == 'foo.en'
 
-    # modify figure_language_filename and language is None
-    app.env.config.language = None
-    app.env.config.figure_language_filename = 'images/{language}/{root}{ext}'
-    assert i18n.get_image_filename_for_language('foo.png', app.env) == 'foo.png'
-    assert i18n.get_image_filename_for_language('foo.bar.png', app.env) == 'foo.bar.png'
-    assert i18n.get_image_filename_for_language('subdir/foo.png', app.env) == 'subdir/foo.png'
-    assert i18n.get_image_filename_for_language('../foo.png', app.env) == '../foo.png'
-    assert i18n.get_image_filename_for_language('foo', app.env) == 'foo'
-
     # modify figure_language_filename and language is 'en'
     app.env.config.language = 'en'
     app.env.config.figure_language_filename = 'images/{language}/{root}{ext}'