Yes they may be appropriate. Sure, a lot of the questions tagged with them will be of the type "O halps! Can you convert these codez from VB.Net to C# for me? Kthxbye!", but there is also likely to be a small subset of good legitimate questions as well.
The way to find out is to trawl the list of questions using them, and flag/close/delete the questions that are poor or not desirable. Once those questions have been actioned by the community or moderators (as applicable), if there are no questions left using the tag then it will automatically be culled from the ecosystem.
While I do think these tags can have a legitimate use, I do believe the wiki description for c#-to-f# needs to have the phrase or learning F# from the perspective of someone with C# background removed - the tag should be for translating C# constructs to F#, not for learning F#.
people, please stop arguing semantics in the comments, I haven't got time for stuff like that. If you think my answer is incorrect please post your own answer stating why these particular tags are worthless. Don't take the "bad until proven good" approach, instead you should do the opposite.
Yes, we know good questions can have bad tags. Yes, we know tags get used incorrectly. That is not in dispute. The question is do language conversion tags have a legitimate use?
So, do the questions currently using these tags need a good clean up? Yes they do, that also isn't in dispute. But you don't just go and burninate (eliminate) a bunch of tags without first showing why they should go - to date nobody has provided any conclusive reasons for this.