Support for BCP47 codes

[jbezos]

There must be a user interface to select languages with the bcp47 code. See plk/biblatex#961 .

There are, however, some loose ends. See for example:

https://developer.android.com/guide/topics/resources/multilingual-support.html?hl=en

http://www.rfc-editor.org/rfc/rfc4647.txt

https://www.w3.org/International/articles/language-tags/

http://www.unicode.org/reports/tr35/

Thus, there are no unique mappings from a set of rules to any identifier, either a language name or a bcp47 code. While en will often be the preferred tag, there are cases where en-Latn can be a better choice (the same applies to regions). Both are legitimate.