Hey Ez, like Beyond Legends has already pointed out, there are actually many vastayan languages (since there are many vastayan tribes), so there's a lot of room for you to apply your own creativity!
I actually helped create the language that Xayah quotes in-game, it's one of the coolest things I've gotten to do here at Riot! I don't think it's Lhotlan though. In fact, I know it's not because it was created for another tribe. [slayer-jinx-wink] Xayah just knows how to speak it, from her travels.
It's actually not that easy to explain how the language works. It's a six page document, and that only covers a small slice of the language; enough for me to write a handful of lines.
Here are some of the things I thought about when creating the language, though (it's baraashi, for the record):
How are sentences structured? In English it's subject -> verb -> object, ex: I, love, cheese ravioli
Baraashi is subject -> object -> verb, ex: I, cheese ravioli, love
How are verbs themselves handled? Conjugation is where a lot of languages get complicated. So for simplicity, I avoided it for the most part (like in Chinese). Words before the verb indicate to who and when the verbs apply. However, there are examples where this changes, a second way to construct sentences that is still correct. ;) Languages are never simple...
How long are words? Compare German to Chinese. Baraashi words should be short, three syllables max.
What sounds should be prevalent? I looked at animals and Finnish as a starting point.
And most importantly for all of the above... why? CertainlyT actually insisted that I sit down and make a bullet point list of what the people speaking the language are like, before anything else.
Anyway, without further ado, here are some sample sentences, translated.
I will gather magic. -> Baasi om soura.
Baasi (I, future)
Om (magic)
Soura (to gather)
They are not-vastaya. -> Faash vastaya na.
Faash (They, present)
Vastaya (vastaya)
Na (To not be)
Vi-om tann va’h.
Vi-om (wild-magic)
tann (here)
va'h (to be, present)
If you didn't recognize it, that last one is a direct Xayah quote. She doesn't pronounce it correctly btw, she has a "human" accent. Which I think says something interesting about the state of vastaya, struggling to stay in touch with their heritage.
tldr, creating languages is actually fun! I apologize for all the times I judged Tolkien for writing elvish poems.