Roll on this table to see what sounds and general feel you should use to make your ancient, magical language sound like. You can connect this to the lore reasons as to why it sounds this bad.
 Ancient Sumerian. Lots of grunting, simple words like UMA or ELE.
 Hebrew. Yelling “Oy Vey!” when your spells fuck up is not necessary, but highly encouraged.
 Ancient Egyptian, or just invoke the names of their Gods since that works too.
 Aztec. Lots of q's and unpronounceable nonsense. Cool art though.
 Just screaming.
 Kulning. Scandinavian cow calling- very fitting for Elvish magic.
 Nonsense rhyming-whyming bullshit words. Bibitties and bobbitties.
 End all the words in exhaling, hissing consonants, like h's and s's. Use for ancient snake sorcerers.
 Say what you want a spell to do and then reverse the tape. Talk backwards.
 Sound effects. Every spell's incantation is just what it sounds like in funny impersonation.
 Tuvan Throat Singing. Can be heard high and far in the mountain steppe.
 Some made up baby speak. Daa daa voo woo. Everything ends in a vowel.
 Sims talk. Use your native language and pick a few words, but make them connect wrongly.
 Dip deep in your throat and make horrible scratchy, growly sounds. Magic words are originally used by demons or other beings, humans aren't well adapted to speak this language.
 Magic words are imperfect recreations of things impossible for humans to pronounce. Words like “Ishjjckkkairruullllklkl” from Eldritch Speech. Or just use Georgian.
 Gather a bunch of spit in your mouth and just make mouth noises instead of speaking. The closest you'll get to actual Sorcerous incantations is Donald Duck speak.
 Latin. You can use Latin literally, since there is good enough representation on how it is spoken, or just use rhyming fake Latin for Harry Potter-esque spell incantations which works too.
 Japanese-y. Give it those loud onomatopoeia like ARA ARA and shit from your favorite anime. Don't assume this is meant to be realistic or anything.
 Spoken in English (or the fantasy version of English), but it always rhymes and makes good use of ancient and forgotten words. The magic language is just the normal one used in this way.