It is not the salah cannot be offered in other language than arabic per se.
It is just that in salah we recite qur'an surahs and verses, which are all in arabic.
we also praise Allah's beautiful names in salah, which are all also in arabic.
On top of that, we must also offer salah just like the prophet Muhammad SAW did.
However, there is opinion that you can make some personal prayers/supplications during sujood or sitting between two sujoods in salah in other languages, but ONLY if you say it BY HEART.