Allow partial locale support

[tarnas14]

Current behaviour:
The localization object is loaded as a whole, based on client's locale preferences, the fallback behaviour works only on the whole localization object level. I.E. when user has locale=es and we don't support es, the localization resources loaded in UI fall back to en (default locale)
Fallbacks do not work on a per key basis - if es localization key is present but does not contain all of the localization keys expected by the application, the application breaks with missing props React error or something similar.

Expected behaviour:
UI localization fallback work on a per key basis so that partial locale support is possible

[kjendrzyca]

Also it would be nice to have some kind of static analysis of the resource files to check if everything was translated.