automatic language code conversion

[evillemez]

It would be nice to have the various codes in the languages property converted automatically to other formats not originally specified.

Some features that would be nice:

• automatically fill in the iso639_3 codes based on the values of the bpc47 codes, and vice versa
• automatically fill in iso639_9 macro lang codes if more specific codes are specified

Mapping configuration for all of this would be very large, but hopefully not difficult to maintain - but, loading it all the time would require not insignificant memory, so, I think it makes sense to do it asynchronously, and not as part of the normal validation process every time a change is made. Depends really on how mapping this stuff gets implemented.