Untranslatable glossary should be per language

### Describe the issue

Marking a glossary term as untranslatable in one language affect all other languages.

One term is untranslatable in one language does not mean it is untranslatable in another. For example,
McDonald's the brand name is almost always untranslatable except in Chinese. This is also true for many tech terms.

Users often set a term to read-only simply because it cannot be translated in their own language. Users of other languages who find that a term they need to translate is marked as read-only will in turn unmark it read-only. It goes back and forth.

### I already tried

- [X] I've read and searched [the documentation](https://docs.weblate.org/).
- [X] I've searched for similar filed issues in this repository.

### Steps to reproduce the behavior

1. Go to some glossary term.
2. Tools > Mark as untranslatable.
3. Check this term in another language.

### Expected behavior

Untranslatable is a per language setting, not per glossary term.