[FEATURE] Adds fallback rules for locale resolution
Allows to set a fallback order for locale resolving. If not set,
the implicit inheritance of locales will be used. That is, if a
locale of en_UK is requested, matches will be searched for in en_UK
and en before trying the default locale configured in FLOW3.
If FLOW3 is given an order of [dk, za, fr_CA] a request for en_UK will
be looked up in en_UK, en, dk, za, fr_CA, fr before trying the default
If strict rule adherence is requested, the above example would instead look
in en_UK, dk, za, fr_CA before trying the default locale. In other words,
the implicit fallback is not applied to the locales in the fallback rule.