I see both Dutch and Dutch (Belgium) are listed here: https://weblate.securedrop.club/projects/securedrop/securedrop/
Some languages differ significantly but in this case the extra workload of providing both nl and nl_BE is a waste of resources. I propose to drop the nl-BE and fallback to nl. This is a lot easier and nothing like az (== az-latn) vs. az-Arab vs. az-Cyrl; or the sr-RU vs. sr.
Fallbacks are hard, users are sometimes (often?) not sending their second or third option, I’d welcome discussion about other languages. Presenting gsw to de users? ro to someone who asks for ru, mo but both ru and mo don’t exist? I presume others have spend more thoughts on this.