Guard against unknown languages attribute values (#17512)

Guard against languages misstype

conans/model/conan_file.py

@@ -95,6 +95,8 @@ def __init__(self, display_name=""):
             self.generators = [self.generators]
         if isinstance(self.languages, str):
             self.languages = [self.languages]
+        if not all(lang == "C" or lang == "C++" for lang in self.languages):
+            raise ConanException("Only 'C' and 'C++' languages are allowed in 'languages' attribute")
         if isinstance(self.settings, str):
             self.settings = [self.settings]
         self.requires = Requirements(self.requires, self.build_requires, self.test_requires,

test/integration/conanfile/conanfile_errors_test.py

@@ -240,3 +240,22 @@ def package_info(self):
     tc.run("create cmake")
     tc.run("create . -b=missing -pr=profile")
     assert "cmake/1.0 -> True" not in tc.out
+
+
+@pytest.mark.parametrize("languages", [
+    "['C', 'CXX']",
+    "['CXX']",
+    "'CXX'",
+])
+def test_language_unexpected(languages):
+    tc = TestClient(light=True)
+    tc.save({"conanfile.py": GenConanfile().with_class_attribute(f"languages = {languages}")})
+    tc.run("inspect .", assert_error=True)
+    assert "Only 'C' and 'C++' languages are allowed in 'languages' attribute" in tc.out
+
+
+def test_empty_languages():
+    tc = TestClient(light=True)
+    tc.save({"conanfile.py": GenConanfile().with_class_attribute("languages = []")})
+    tc.run("inspect .")
+    assert "Only 'C' and 'C++' languages are allowed in 'languages' attribute" not in tc.out