Clear licenses for translations in weblate

Our free account in weblate depends on the translations made for Free and Libre software.

We do not have a clear policy about licenses for translations, and if possible decide on a default license for them.

The licenses offered in Weblate are explained here: https://spdx.org/licenses/.

We need to find out which licenses the translations should have, fix the licenses on weblate, and document the reasoning.