Support fallback to the "generic" language variant

**Is your feature request related to a problem? Please describe.**
We decided to use a fallback strategy to limit the amount of duplicate translations for similar languages in our application, this means that several country-specific language files are now almost empty (e.g. `nl-BE` contains only specific overrides on `nl`).
Weblate is showing a lot of untranslated strings as part of this (and also doesn't seem to have an easy method to _remove_ a duplicate)

**Describe the solution you'd like**
Allow an option/flag to take into account the _base language_ (i.e. `nl`) when providing insights for a country-specific variant (like `nl-BE`).
I believe that this should ideally mark translations as needing verification, and then marking it as translated internally (without adding the key to the country-specific file)

**Describe alternatives you've considered**
Using secondary language to at least see them, however the _secondary language_ will depend on which language is being translated, additionally this shows a lot of keys as not translated, which can be confusing.

**Additional context**