Warn against translating to redundant languages

We regularly get requests of the form "I've got both <ll> and <ll>_<CC> translations" (where ll is a language code and CC the language's "home" region/country code) "so please delete the second language."

We can't just delete a language, only the redundant translations. That still takes engineering time as well as management approval. And after that, the bad translations may well come back. To put a stop to this, the translation pages for <ll>_<CC> should warn against translating there, and refer the user to the <ll> pages instead.

Nasty cases that complicate the algorithm:

 * pt_BR users must not be pointed to pt, nor nl_BE users to nl etc.

 * There is no zh, and so zh_CN should never refer users to it.

 * The default for en is en_US, not en_GB.

So the system must know that <CC> really is <ll>'s default region/country code; that would require extending the language information tables.

Or alternatively, the system should be able to guess the default code without false positives (maybe just the cases where the region code matches the language code, e.g. ru_RU and pt_PT?), and take care not to refer from a visible language (zh_CN) to a non-visible one (zh).