Fix automatic language detection on CS:GO (bug 6163).

As noted in [bug 6163](https://bugs.alliedmods.net/show_bug.cgi?id=6163), cl_language is no longer available as a userinfo var in CS:GO, breaking our automatic client language detection.

However, it does still exist, albeit hidden on the client. We are able to query the value.

There are some drawbacks to this compared to getting it from userinfo (inconsistent behavior across games, language not being available until shortly after PutInServer rather than Connect on CS:GO, extra logic running for every convar query in CS:GO), but I think that it's still better than nothing.