Because the translations of all the "main" packages (and a few others) are brought by the language packs, and also MATE users most likely use some "main" packages. He is simply supposed to have the language packs for his language installed. That's how Ubuntu including the flavors has been designed for many cycles.
Since we are close to the release of an LTS, I want to narrow the fix to only affect where it matters, i.e. the live sessions.
Because the translations of all the "main" packages (and a few others) are brought by the language packs, and also MATE users most likely use some "main" packages. He is simply supposed to have the language packs for his language installed. That's how Ubuntu including the flavors has been designed for many cycles.
Since we are close to the release of an LTS, I want to narrow the fix to only affect where it matters, i.e. the live sessions.