Shulchan Aruch, Orach Chayim 101:4:
One may pray in any language one wishes…
Mishnah Berurah, ad loc:
This applies only on a non-permanent basis, however is completely forbidden to establish as permanent practice [to pray in a language other than Hebrew] and to appoint a prayer leader (שליח ציבור) [to lead the service in a language other than Hebrew], and [thereby] to cause the Holy Tongue to be completely forgotten among Israel. This stands in counterpoint to the new sects that have arisen… that have translated the entire text of prayer into a gentile language. One transgression leads to another, for they have omitted the blessing regarding the ingathering of the exiles, and the blessing “And to Jerusalem, your city” (regarding the rebuilding of Jerusalem), and just as they desire to cause the memory of Jerusalem to be forgotten, so do they desire that the Holy Language be forgotten from Israel, lest they be redeemed in the merit of not deviating from their [Hebrew] language. May the Holy One Blessed is He protect us from such heretical thoughts!