The consensus is that public prayer can be in any language, but private prayer must be in Hebrew. Below is a compendium of what various commentators said:
Mishnah Sotah 7:1
These are said in any language: The portion of the Sotah, the confession over tithes, the reading of the Shema, tefillah [Amida], the grace after meals, the oath relating to testimony and the oath relating to a bailiment.
And these are said in the holy language [Hebrew]: The reading over first-fruits, the release from levirate marriage, the blessings and curses [recited by Israel upon entering the land], the priestly blessing, the blessing of the high priest, the portion of the king, the portion of the broken heifer, and the speech of the priest anointed for battle to the people.
Talmud Yerushalmi Sotah 7:1, 21b
The reading of the Shema [can be done in any language]. Because it is written: “And you shall speak them.” Rabi says: “I say that the Shema can only be read in Hebrew.” Why? “And these words [that I command you today shall be in your mind].
R. Levi bar Ḥaita went to Caesarea. He heard voices reading the Shema in Greek. He wanted to stop them. R. Yose heard this and got angry. He said: “Here is what I am saying, should a person who doesn’t know Hebrew not say the Shema at all? No, one’s obligation is fulfilled in whatever language one knows… Prayer [can be said in any language]. So that a person will know how to ask for his/her needs. The Grace after Meals [can be said in any language]. So that one will know whom
one is blessing.
Talmud Bavli Sotah 33a
Tefillah. It is a request for mercy; one prays however one needs to. And can tefillah indeed be in any language? Did not R. Yehudah say: One should never ask for one’s needs in Aramaic, for R. Yoḥanan said: Anyone who asks for his needs in Aramaic is not attended to by the ministering angels, because the ministering angels do not understand Aramaic! There is no contradiction [between the Mishnah and R. Yehudah’s statement]; [R. Yehudah’s ruling applies in] private, [the Mishnah’s applies in] public
Rif Berakhot 7a
That which it says, “tefillah may be recited in any language,” only applies in public, but in private, it is not applicable, for R. Yehudah said in the name of Rav: One should never ask for one’s needs in Aramaic, and R. Yoḥanan said: Anyone who asks for his needs in Aramaic is not attended to by the ministering angels, because the ministering angels do not understand Aramaic.
Rabbeinu Yonah on the Rif vs Gemara
Since we have concluded that private tefillah can only be said in Hebrew, I am deeply puzzled by the universal custom of women to pray in other language, since by virtue of their obligation in prayer, they should have to pray in Hebrew [like everyone else]. The rabbis of France try to justify this practice by saying that when an individual prays the same tefillah that is recited in public, [that private prayer] is considered “public”, and one can say it in another language. And that which R. Yehudah said, “One should never ask for one’s needs in Aramaic, applies when one asks for one’s private needs, such as praying for a sick person or regarding any other problem in one’s home, but tefillah, which is known by the whole community, even when one prays it in one’s own home, it is as if it is being prayed in public, and if one does not know Hebrew, one may fulfill one’s obligation in
any language. And the reason that public requests are made in any language but private needs are not is because the community [when praying as a whole] does not need an intermediary with God, but an individual does…and the ministering angels only attend to [requests made in] Hebrew.
Rosh Berakhot 2:2
It seems to me that [the practice cited by R. Yonah] poses no difficulty, because it was specifically in [Aramaic] that R. Yehudah forbade making requests.
Shulḥan Arukh Orah Ḥayyim 101:4
1. One may recite tefillah in any language one chooses. This, however, is true only in public, but in private, one may only recite tefillah in Hebrew. (Rif)
2. Some say that [the ban on other languages] applies only to asking for one’s [personal] needs, such as praying for a sick person or regarding any other problem in one’s home, but the fixed tefillah recited in public may even be recited privately in any language. (Rabbis of France)
3. Some say that even an individual making private requests may do so in any language he chooses, except for Aramaic. (A lenient reading of Rosh)
Sefer Ḥasidim 588, R. Yehudah HeḤasid et al, Germany, 12th- 13th c.
If you come across a man who does not understand Hebrew, and he is God-fearing and wshes to have proper intent, or a woman, tell them that they should learn the prayers in the language they understand, because prayer is only effective with an understanding mind, and if the mind does not understand, what possible effect could the words have when they come out of one’s mouth? Therefore, it is better to pray in the language that one understands.
Synopsis of Ḥatam Sofer 6:84, R. Moshe Sofer, Hungary, 19th c.
Ruling: The permission to pray in a language other than Hebrew only applies on an ad hoc basis; one may not permanently have prayers in other languages. Reasons Given:
The Men of the Great Assembly fixed the prayers in Hebrew, even though many people at the time did not speak Hebrew.
Prayer was meant, in part, to address the absence of the Temple, and the Men of the Great Assembly chose the Hebrew words carefully in order to achieve these goals.
Hebrew captures nuances and allusions that other languages do not.
If the concern is that people do not understand, it is best to leave the prayers in Hebrew and to educate people, rather than to change the language of prayer.
Hebrew is God’s language, used to address the prophets and to create the world. Anyone who deviates from established custom has the burden of proof.