Move nl_NL to nl localization
There was another request to get nl_BE for bitmask_android in transifex. There's a working fallback to nl_NL for people who have their device configured to nl_BE. Reviewing, extra translations for locales that hardly differ is energy that could be better spent on improving the existing strings (or elsewhere). I propose to change the nl_NL to nl to make sure that those are as 'pan-dutch' as possible.
Arguments a. we haven't had complaints from nl_BE or nl_SR (or..) speakers (same for other projects like Securedrop), b. Dutch a 'language union' (Flanders, Suriname, Netherlands and South-Africa) that tries to standardize written language c. there are no relevant local strings (like €/$)
I've discussed this with l10nlabs too. If there are no objections, I'll figure out how to do this change.