Can someone please add the appropriate synonym?
These tags should not be made synonyms. They appear to be two different tags (although frequently used incorrectly/interchangeably). One is for the use of language and the other is for questions about non-English languages. I'm not sure which should be which, but first I would add/correct the tag wikis to properly explain the use case, and then we'll need a bit of a tag cleanup to make their usage consistent.
But as a side note, even if these tags meant the same thing, I wouldn't recommend adding a tag synonym to handle pluralization as such. Tag synonyms were designed to be used when there are two completely different words used for the same concept. Think "cars" versus "autos". But for something like this, text completion would help the user pick the correct usage. lan → language.
Apparently these tags are supposed to reflect 2 very different concepts, although that intended usage is not clear from the current usage nor from the tag wikis.
Since tags names should not invite confusion (otherwise you end up with a jumbled mess like we have now), both should be renamed to eliminate the confusion.
As such, I propose:
I am not crazy about the naming of "foreign languages" as that has a decidedly pro-English context but I could not think of another short choice of words that would accurately describe the concept.
Likewise, it looks like there is a 3rd use case mixed in there and that is programming language. As that tag already exists, we just need to retag those questions.
And the original tag names should be made synonyms of the new tag names once everything has been retagged appropriately.