Move message formatter in LanguageEngine, add locale for message formatter

[flatron4eg]

Added value for locale parameter for MessageFormatter, as plural categories ("one", "few", "other") work differently for different locales. Also refactored LanguageEngine a bit, so now MessageFormatter is part of it.

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[marticliment]

I want to check the changes carefully, but it looks good at a first glance.

I will check and merge the PR in 3/4 days