depends a little on context. Is this referring to a conversation going down hill, or the grammar of a culture in decline? . fall apart, be forgotten fail them become more diverse become mixed with other influences be lost be less heard in common speech loose its cohesion . Mr. Google says worsen, get worse, decline, be in decline, degenerate, decay; collapse, fail, fall, drop, sink, slump, slip, slide, go downhill, go backwards, go to rack and ruin, stagnate, wane, ebb; informalgo to pot, go to the dogs, hit the skids, go down the toilet, go down the tubes; informalgo to the pack; rareretrograde decay, degrade, degenerate, break down, decompose, rot, putrefy, go bad, go off, spoil, perish; wither, atrophy, weaken, fade, break up, disintegrate, become dilapidated, crumble, fall down, collapse, fall apart, fall to pieces; archaiccorrupt

Hello. Thank you very much! These are actually formal synonyms that I am looking for :).

Hi Magdalena! You could say that "the language has started to fall apart." Get worse is als an alternative. For example, "the language has gotten worse over the last 20 years." Note that "deteriorate" has a connotation of formality; the options I gave you are less formal.