LaTeX not well configured for Greek language as main language

[jfbu]

Describe the bug

When using language = 'el' and using latexpdf, the PDF will be ok only if entirely in Greek. Any Latin letter will become converted into a Greek letter. Besides there are problems with Unicode letters even as common as © or ™, due to a current bug in LaTeX package babel for non-English support.

Thus, Sphinx should use xelatex per default.

However there is currently an issue in LaTeX package polyglossia which requires to incorporate a workaround.

And our documentation should explicitly state that the advices for supporting small Greek snippets do not apply when language is 'el'.

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]
• Python version: [e.g. 3.7.1]
• Sphinx version: 2.0+
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: TeXLive2019

Additional context

There was discussion about this at the sphinx-users mailing list