Change the language type given to CatalogRepository in tests

[adamtheturtle]

Some tests gave None as the language.
The CatalogRepository signature says that language should be a str:

class CatalogRepository:
    """A repository for message catalogs."""

    def __init__(self, basedir: str | os.PathLike[str], locale_dirs: list[str],
                 language: str, encoding: str) -> None:

I also looked at whether the language type should be changed to str | None. I believe that it should not - outside of tests, the only callers (within Sphinx) of CatalogRepository.__init__ pass in self.config.language, where config is a sphinx.config.Config.

Looking at that, I see:

...

config_values: dict[str, _Opt] = {
    ...
    'language': _Opt('en', 'env', frozenset((str,))),
}

This shows that the valid types at the config level do not include types.NoneType.