Uppercase letters in language names are unsupported

[iafan]

If one defines a language with uppercase text in it, e.g. de-DE, then Serge can't properly find a translation in a database.

While it's recommended to use ISO language codes (which are lowercase), Serge should support any kinds of strings as language names.

[julen]

For completeness, here's a relevant excerpt from IETF's BCP 47 on Language Tags:

Although case distinctions do not carry meaning in language tags,
consistent formatting and presentation of language tags will aid
users. The format of subtags in the registry is RECOMMENDED as the
form to use in language tags. This format generally corresponds to
the common conventions for the various ISO standards from which the
subtags are derived.

These conventions include:

o [ISO639-1] recommends that language codes be written in lowercase
('mn' Mongolian).

o [ISO15924] recommends that script codes use lowercase with the
initial letter capitalized ('Cyrl' Cyrillic).

o [ISO3166-1] recommends that country codes be capitalized ('MN'
Mongolia).

An implementation can reproduce this format without accessing the
registry as follows. All subtags, including extension and private
use subtags, use lowercase letters with two exceptions: two-letter
and four-letter subtags that neither appear at the start of the tag
nor occur after singletons. Such two-letter subtags are all
uppercase (as in the tags "en-CA-x-ca" or "sgn-BE-FR") and four-
letter subtags are titlecase (as in the tag "az-Latn-x-latn").