i18n fallback fix

this allows
- `be` to fallback to `be-BY`   (belerusian)
- `pt` to fallback to `pt-BR`   (portugeuse)

the solution comprises a few parts:
- adding a stub `fallback.be.yml` so that i18n auto-includes `be` in `available_locales`
- adding a constant to the locales_helper to track fallback locales
- adding switch in the method `locale_name` so that `be` knows what to call itself and doesn't just offer ` see this website in ------ `  in frontpage locale banner

this last one could be done differently, open for feedback
This is the only bit I didn't like 
If I've understood, the require is run first in the initializer, which makes LANGUAGES a global const. 
What I didn't understand was why the original code didn't work here - we've required the variables into the LocalesHelper and don't need the `::LANGUAGES` form within LocalesHelper, but that hasn't made the constants belong to the LocalesHelper?

that's a theory question which you're not obliged to answer.