There was a Feature Request 0012334: Post-install language selection & dictionary updating: created last June about making this easier and uncoupling Language updates from the Gramps main program update cycle. It has been acknowledged but no one has adopted that project yet.
Unfortunately, I know only a little about changing languages in the PortableApps port of Gramps. So I would also appreciate any information shared here about:
- managing translation language files, dictionaries, & affix files
- adding a language without re-installing
- switching between languages for the GUI & reports
- hacking the language files locally or via Weblate