I used to do quite a bit, though mainly starting with a handful of vocabulary and a fairly well fleshed out grammar, then throwing in new vocab as needed. The big problem with that was that, while I enjoyed doing it, it was mostly wasted effort since most players imxp don't to bother with elaborate placenames, titles, translation errors or the like.
And of course, more recently real life allows less time, and the internet has made it almost trivial to access real languages, so fully inventing new languages now does little more than take time away from other world-building tasks. Nowadays I restrict my in-world languages to be real-world languages, possibly slightly twisted by grammar, vocab and pronunciation quirks, and written in invented alphabets.