Sefer Chasidim: Pray In Your Language

If God-fearing people who do not understand Hebrew ask you how they should say their prayers, tell them to learn to recite the prayers in the language they understand. Because prayer depends on knowing what you are saying, and if your mind does not comprehend the words you are uttering, what is the use of praying? Therefore, it is better that people should pray in the language they understand. – Sefer Chasidim #588, Translated by Avraham Yaakov Finkel. (See Magen Avraham 101:5, Teshuvot Rivash 388-391)