I had an occasion to congratulate someone, but Nevashi came up short. I have since remedied that.
First, I looked up the etymology of "congratulation" and then searched out and considered how other languages handle congratulations. In the end, I found that I already had the words I wanted: Anta jeya! ("There is happiness!" or maybe "It is a joy!")
So, there is no verb "to congratulate", but you might say something like, Za twen ea-jeyam voz. (1st+past+give share happiness-acc his/her.), "I shared his or her happiness.") or perhaps just ja tel vosh, "anta jeya" (1st-past-speech speak (3rd person-epicene-dative-pronoun), "There-is happiness".; "I told him/her "congratulations".)
A note in passing...
There are not separate words for 'speak', 'talk', and 'say' in Nevashi at this point. There are variations on the one word, tel, that mean to talk loudly, to whisper, or to talk a lot, and there are different words that specifically mean "shout" (magan), "tell a story, recite, or make a speech" (hlal) and "pray" (nedh), but there is yet no distinction as made in English for the three words above. I think it might stay that way unless I see a pressing need to change that.
There are other parts of even the limited vocabulary it has already developed that are a lot more specific than English. (See gwelva, the variations on "love", and/or the words that might be covered by "healthy" in English.)