Locale fallback

[c960657]

This PR is a follow-up to #123.

This PR adds support for fallback locales. This has two major purposes:

1. The available translations are rather sparse, i.e. each provider only has full support for a few locales. When using a locale that is not fully supported, you may want to try a number of locales before falling back to English. It depends e.g. on whether your users are likely to understand the local language.
2. Allowing fallback from e.g. en_US to en introduces a kind of inheritance, so that strings shared between English variants are represented only once, while still allowing override for specific countries. This allows us to provide translations for all 102 en_* locales without repeating same same strings 102 times. This change makes the translation files less redundant, and we support a lot more locales.

Regarding #2: I removed duplicate entries for existing translations (e.g. if en_US, en_GB and all other variants were identical, I simply replaced them with one en entry). If there was a difference between e.g. de_DE and de_AT, I generally chose the larger country as the general translation (i.e. the translation for de_DE becomes de). This is somewhat arbitrary, but it can easily be changed later on without changing the currently supported locales (it only affects locales that are not currently supported).

[c960657]

Finally got around to updating the translations inlined into the national providers.

[c960657]

Closing in favour of #176 (too many merge conflicts and slightly different approach).