What does it take for a language to become a language? Quite simply: grammar. Any constructed language in fiction has the potential to become functional languages, all it takes is a sound structure that is productive and consistent. It’s not the size of the lexicon but the ability to distill fundamental rules about how things such as plurals, tenses, cases, progressives, etc. can be consistently realised in the language.
Thus being able to speak Quenya or Sindarin, some of the most comprehensive constructed languages out there, shouldn’t be something of a shame. J.R.R. Tolkien has in fact said before, “The ‘stories’ were made rather to provide a world for the languages than the reverse.”
There are criticisms that constructed languages cannot be compared at the same level of languages that exist in real life, and that the ‘evolution’ that languages such as Quenya experiences in the literature isn’t the same as when real-life languages develop regional dialects and accents from contact with other people and isolation from geographical boundaries. Why should that be the case? Just simply because the change is the result of an author’s machinations doesn’t discredit any change ascribed to the constructed language from being any less real, if said constructed languages mimic real-life examples of language evolution.
There are several constructed languages in the world that experience little to no regional change, such as Esperanto or Lojban, but why should they be seen as more legit than Klingon, simply because people have had the opportunity to use them in real life?
So go forth and proudly claim your multilingualism!