Support dumping translations for a locale and the fallback locale in the same file

[stof]

currently, when using dumped translations, we have to load 2 files: the one for the current locale, and the one for the fallback locale (otherwise fallback feature is not working fine).

It would be great if dumped translation files could include the fallback translations too, to allow loading a single file (this should be opt-in). Note that we could optimize file size by not dumping the whole fallback translations, but only the one which are not part of the main catalogue (i.e. the ones which will fallback)

[monteiro]

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