כיון שגלו ישראל בימי נבוכדנאצר הרשע, נתערבו בפרס ויוון
ושאר האומות, ונולדו להם בנים בארצות הגויים; ואותן הבנים נתבלבלה שפתם,
והייתה שפת כל אחד ואחד מעורבת מלשונות הרבה. וכיון שהיה מדבר, אינו יכול
לדבר כל צרכיו בלשון אחת אלא בשיבוש, שנאמר "ובניהם, חצי מדבר אשדודית,
ואינם מכירים, לדבר יהודית--וכלשון, עם ועם" (נחמיה יג,כד).
ומפני זה, כשהיה אחד מהן מתפלל, תקצר לשונו לשאול חפציו או
להגיד שבח הקדוש ברוך הוא בלשון הקודש, עד שיערב עימה לשונות אחרות.
וכיון שראה עזרא ובית דינו כך, עמדו ותיקנו להם שמונה עשרה ברכות על הסדר.
Once the Jewish people were exiled during the times of the evil Nebuchadnezzar, they mixed with the Persians, Greeks and other nationals, having children with them in the foreign lands. [Rambam is saying that intermarriage was common]. Those children’s language became confused. The language of each of them became suffused with many idioms and when speaking, they would express themselves erroneously as it says, “Their children, half spoke the language of Ashdod and the language of those various people, not knowing how to speak Judean”. For that reason, when one of them would pray, his vocabulary was lacking and would not allow him to express himself to ask for his needs or to praise HKBH in the Holy Language (verbatim: Language that emanates from the Holy) without mixing in other idioms. When Ezra and his Beit Din saw this, they instituted for them the Eighteen Blessings in their proper order.
Rambam, in his Pirush Hamishna Sotah 7:1 on the Mishna that allows one to pray in any language, limits that latitude to public prayer. For private prayer, one should endeavor to use Hebrew. He does not legislate this in Mishne Torah. The sense I get here is that the objection is for mixing languages. Be it as it may, the old process of spontaneous prayer became obsolete and a new way of performing the Mitzvah of Tefilah as Avodah had to be put in place. Clearly though, spontaneity is the preferred method and formulaic prayer was only instituted in reaction to circumstances.
It is important to always keep in mind that according to Rambam all Mitzvot that require action to perform them are concessions to our humanity. The preferred method of worship is intellectual without any physical expression. We as humans cannot reliably perform this type of worship and we need to have ways of expressing our intellectual apprehension and the resulting emotions and feelings that we experience. Leaving this to each one of us is dangerous as our imagination takes over and inevitably we will find ourselves worshipping in ways that will lead to idolatry and the belief in the supernatural. Practice has a way of reflecting back onto the intellect and misleading it. That is what happened to humanity historically and is the source of all the myths invented by early man. The Torah recognizes this human trait and strictly regulates how one can express these feelings. It however has to walk a fine line between regulation and the risk of killing off spontaneous worship. It is in this spirit that prayer was formulated as praise followed by supplication and finally acknowledgement. The wording however was left to the individual until that became problematic. Using languages other than Hebrew allows for nuances that compromise the proper way of thinking about God and may lead to erroneous theology. That is how Meiri explains the Gemara Shabbat 12b that says that Mal’achei Hasharet, angels do not know Aramaic. Angels are a metaphor for the human intellect. Using an unknown language in worship distorts the meaning and eventually will reflect back onto the intellect. The Rabbis therefore felt that it was not enough to regulate the content in general terms but there was also a need to develop a formula that would address the praise section followed by a broad enough formula to cover many possibilities in the supplication section. This was done at the risk of dampening spontaneous expression. In fact deviating from the formula now is seen as an error. To be continued…
 Rav Kafieh in his notes on this Halacha disagrees. I find his argument apologetic.
 Based on the Gemara Sotah 33a.
 Chibur Hateshuvah page 510.