diff --git a/docs/pages/release_notes.md b/docs/pages/release_notes.md
index 5f4f460f3f1..acd1d647869 100644
--- a/docs/pages/release_notes.md
+++ b/docs/pages/release_notes.md
@@ -18,6 +18,8 @@ This is a {{ site.pmd.release_type }} release.
 
 * apex
     * [#4096](https://github.com/pmd/pmd/issues/4096): \[apex] ApexAssertionsShouldIncludeMessage and ApexUnitTestClassShouldHaveAsserts: support new Assert class (introduced with Apex v56.0)
+* core
+    * [#3970](https://github.com/pmd/pmd/issues/3970): \[core] FileCollector.addFile ignores language parameter
 * java-codestyle
     * [#4082](https://github.com/pmd/pmd/issues/4082): \[java] UnnecessaryImport false positive for on-demand imports of nested classes
 
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/NioTextFile.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/NioTextFile.java
index c7d77e1de88..16ab46f93ff 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/NioTextFile.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/NioTextFile.java
@@ -18,6 +18,7 @@
 import net.sourceforge.pmd.util.IOUtil;
 import net.sourceforge.pmd.util.datasource.DataSource;
 import net.sourceforge.pmd.util.datasource.FileDataSource;
+import net.sourceforge.pmd.util.datasource.internal.LanguageAwareDataSource;
 
 /**
  * A {@link TextFile} backed by a file in some {@link FileSystem}.
@@ -73,7 +74,7 @@ public String readContents() throws IOException {
 
     @Override
     public DataSource toDataSourceCompat() {
-        return new FileDataSource(path.toFile());
+        return new LanguageAwareDataSource(new FileDataSource(path.toFile()), languageVersion);
     }
 
     @Override
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/StringTextFile.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/StringTextFile.java
index 598823e555c..c4d286d8950 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/StringTextFile.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/document/StringTextFile.java
@@ -12,6 +12,7 @@
 import net.sourceforge.pmd.lang.LanguageVersion;
 import net.sourceforge.pmd.util.datasource.DataSource;
 import net.sourceforge.pmd.util.datasource.ReaderDataSource;
+import net.sourceforge.pmd.util.datasource.internal.LanguageAwareDataSource;
 
 /**
  * Read-only view on a string.
@@ -64,9 +65,10 @@ public String readContents() {
 
     @Override
     public DataSource toDataSourceCompat() {
-        return new ReaderDataSource(
+        return new LanguageAwareDataSource(new ReaderDataSource(
             new StringReader(content),
-            pathId
+            pathId),
+            languageVersion
         );
     }
 
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/processor/PmdRunnable.java b/pmd-core/src/main/java/net/sourceforge/pmd/processor/PmdRunnable.java
index da91fbd95a6..e4b1d75de62 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/processor/PmdRunnable.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/processor/PmdRunnable.java
@@ -21,6 +21,7 @@
 import net.sourceforge.pmd.benchmark.TimeTracker;
 import net.sourceforge.pmd.renderers.Renderer;
 import net.sourceforge.pmd.util.datasource.DataSource;
+import net.sourceforge.pmd.util.datasource.internal.LanguageAwareDataSource;
 
 /**
  *
@@ -82,6 +83,9 @@ public Report call() {
 
         try (InputStream stream = new BufferedInputStream(dataSource.getInputStream())) {
             tc.ruleContext.setLanguageVersion(null);
+            if (dataSource instanceof LanguageAwareDataSource) {
+                tc.ruleContext.setLanguageVersion(((LanguageAwareDataSource) dataSource).getLanguageVersion());
+            }
             sourceCodeProcessor.processSourceCode(stream, tc.ruleSets, tc.ruleContext);
         } catch (PMDException pmde) {
             addError(report, pmde, "Error while processing file: " + fileName);
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/util/datasource/internal/LanguageAwareDataSource.java b/pmd-core/src/main/java/net/sourceforge/pmd/util/datasource/internal/LanguageAwareDataSource.java
new file mode 100644
index 00000000000..407001e1028
--- /dev/null
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/util/datasource/internal/LanguageAwareDataSource.java
@@ -0,0 +1,42 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.util.datasource.internal;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+import net.sourceforge.pmd.annotation.InternalApi;
+import net.sourceforge.pmd.lang.LanguageVersion;
+import net.sourceforge.pmd.util.datasource.DataSource;
+
+@InternalApi
+public class LanguageAwareDataSource implements DataSource {
+    private final DataSource base; // delegate DataSource methods to this
+    private final LanguageVersion version;
+
+    public LanguageAwareDataSource(DataSource base, LanguageVersion version) {
+        this.base = base;
+        this.version = version;
+    }
+
+    public LanguageVersion getLanguageVersion() {
+        return version;
+    }
+
+    @Override
+    public InputStream getInputStream() throws IOException {
+        return base.getInputStream();
+    }
+
+    @Override
+    public String getNiceFileName(boolean shortNames, String inputFileName) {
+        return base.getNiceFileName(shortNames, inputFileName);
+    }
+
+    @Override
+    public void close() throws IOException {
+        base.close();
+    }
+}
diff --git a/pmd-core/src/test/java/net/sourceforge/pmd/PmdAnalysisTest.java b/pmd-core/src/test/java/net/sourceforge/pmd/PmdAnalysisTest.java
index 4ec5555cf97..a1f2f0f1569 100644
--- a/pmd-core/src/test/java/net/sourceforge/pmd/PmdAnalysisTest.java
+++ b/pmd-core/src/test/java/net/sourceforge/pmd/PmdAnalysisTest.java
@@ -14,10 +14,18 @@
 import static org.mockito.Mockito.verify;
 
 import java.io.IOException;
+import java.nio.file.Paths;
+import java.util.List;
 
+import org.junit.Assert;
 import org.junit.Test;
 import org.mockito.ArgumentMatchers;
 
+import net.sourceforge.pmd.lang.Dummy2LanguageModule;
+import net.sourceforge.pmd.lang.Language;
+import net.sourceforge.pmd.lang.ast.Node;
+import net.sourceforge.pmd.lang.document.SimpleTestTextFile;
+import net.sourceforge.pmd.lang.rule.AbstractRule;
 import net.sourceforge.pmd.renderers.Renderer;
 
 /**
@@ -72,4 +80,54 @@ public void testRulesetWhenSomeoneHasAnError() {
         }
     }
 
+    @Test
+    public void testFileWithSpecificLanguage() {
+        final Language language = Dummy2LanguageModule.getInstance();
+        PMDConfiguration config = new PMDConfiguration();
+        config.setIgnoreIncrementalAnalysis(true);
+        RuleSet ruleset = RuleSet.forSingleRule(new TestRule());
+
+        try (PmdAnalysis pmd = PmdAnalysis.create(config)) {
+            pmd.addRuleSet(ruleset);
+            pmd.files().addFile(Paths.get("src", "test", "resources", "sample-source", "dummy", "foo.txt"), language);
+            Report report = pmd.performAnalysisAndCollectReport();
+            for (Report.ProcessingError error : report.getProcessingErrors()) {
+                System.out.println("error = " + error.getMsg() + ": " + error.getDetail());
+            }
+            Assert.assertEquals(0, report.getProcessingErrors().size());
+            Assert.assertEquals(1, report.getViolations().size());
+        }
+    }
+
+    @Test
+    public void testTextFileWithSpecificLanguage() {
+        final Language language = Dummy2LanguageModule.getInstance();
+        PMDConfiguration config = new PMDConfiguration();
+        config.setIgnoreIncrementalAnalysis(true);
+        RuleSet ruleset = RuleSet.forSingleRule(new TestRule());
+
+        try (PmdAnalysis pmd = PmdAnalysis.create(config)) {
+            pmd.addRuleSet(ruleset);
+            pmd.files().addFile(new SimpleTestTextFile("test content foo", "foo.txt", "foo.txt", language.getDefaultVersion()));
+            Report report = pmd.performAnalysisAndCollectReport();
+            for (Report.ProcessingError error : report.getProcessingErrors()) {
+                System.out.println("error = " + error.getMsg() + ": " + error.getDetail());
+            }
+            Assert.assertEquals(0, report.getProcessingErrors().size());
+            Assert.assertEquals(1, report.getViolations().size());
+        }
+    }
+
+    public static class TestRule extends AbstractRule {
+        public TestRule() {
+            setLanguage(Dummy2LanguageModule.getInstance());
+            setMessage("dummy 2 test rule");
+        }
+
+        @Override
+        public void apply(List<? extends Node> nodes, RuleContext ctx) {
+            ctx.addViolation(nodes.get(0));
+        }
+    }
+
 }
diff --git a/pmd-core/src/test/java/net/sourceforge/pmd/lang/Dummy2LanguageModule.java b/pmd-core/src/test/java/net/sourceforge/pmd/lang/Dummy2LanguageModule.java
index e5a8de77cb6..9d71d220101 100644
--- a/pmd-core/src/test/java/net/sourceforge/pmd/lang/Dummy2LanguageModule.java
+++ b/pmd-core/src/test/java/net/sourceforge/pmd/lang/Dummy2LanguageModule.java
@@ -16,4 +16,8 @@ public Dummy2LanguageModule() {
         super(NAME, null, TERSE_NAME, "dummy2");
         addVersion("1.0", new DummyLanguageModule.Handler(), true);
     }
+
+    public static Language getInstance() {
+        return LanguageRegistry.getLanguage(NAME);
+    }
 }
diff --git a/pmd-core/src/test/java/net/sourceforge/pmd/lang/document/SimpleTestTextFile.java b/pmd-core/src/test/java/net/sourceforge/pmd/lang/document/SimpleTestTextFile.java
new file mode 100644
index 00000000000..6f663471427
--- /dev/null
+++ b/pmd-core/src/test/java/net/sourceforge/pmd/lang/document/SimpleTestTextFile.java
@@ -0,0 +1,17 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.document;
+
+import net.sourceforge.pmd.lang.LanguageVersion;
+
+/**
+ * Makes {@link StringTextFile} publicly available for unit tests.
+ */
+public class SimpleTestTextFile extends StringTextFile {
+
+    public SimpleTestTextFile(String content, String pathId, String displayName, LanguageVersion languageVersion) {
+        super(content, pathId, displayName, languageVersion);
+    }
+}
diff --git a/pmd-core/src/test/resources/sample-source/dummy/foo.txt b/pmd-core/src/test/resources/sample-source/dummy/foo.txt
new file mode 100644
index 00000000000..adaad871778
--- /dev/null
+++ b/pmd-core/src/test/resources/sample-source/dummy/foo.txt
@@ -0,0 +1 @@
+A dummy file with file extension txt.