As you are trying to create a synonym for loan-words, it's for that tag that you should have the required score.
I agree that those tags should be synonym. While borrowing is the "process" that creates loan words, I don't see any reason for having two separated tags.
Creating a question on the meta site is the right do to do.
If there are enough users with the required score on loan-words, and they vote the synonym because they read this question, then the synonym will be automatically created. If there aren't enough users with the required score, moderators can consider creating the tag synonym, basing on the votes this question gets.
Even in the case the tag synonym is created because it get enough votes, there could still be the need of a moderator action. Creating a synonym doesn't automatically rewrite all the existing questions that use the old tag; only a moderator can merge two tags, and when that is done the questions using the old tag are changed to use the new one.
This means that writing a request about tag synonyms here is probably needed in most of the cases, except when you don't mind seeing the old questions using the old tag, and the new questions using the new tag.