Custom Translations for Languages other than en_US

Hi there!
I was trying to override the translation in German for the Field Group “Social”. In german it’s translated to “Sozial” but I would rather like it to say “Marketing”. There’s already some fields associated to the group Social so creating a new group and moving all those fields to the new group would be a bit more effort I guess.

I know overriding the translation files for en_US is very easy by just using the same key in the messages.ini file. But that does not seem to be working for the german messages.ini file.

Here’s a short description of how my files look like:
plugin-name/Translations/en_US/messages.ini

mautic.lead.field.group.social="Marketing"

plugin-name/Translations/de/messages.ini

mautic.lead.field.group.social="Marketing"

I have no experience with Transifex but I assume it is for general translation, not custom ones, right?

Thanks for your help.