Siman 101ק״א
1 א

שצריך לכוין בכל הברכות ושיכול להתפלל בכל לשון. ובו ד סעיפים:
המתפלל צריך שיכוין בכל הברכות ואם אינו יכול לכוין בכולם לפחות יכוין באבות אם לא כיון באבות אע"פ שכיון בכל השאר יחזור ויתפלל: הגה (והאידנא אין חוזרין בשביל חסרון כוונה שאף בחזרה קרוב הוא שלא יכוין אם כן למה יחזור) (טור):

That one must have the proper intent for all the blessings and that one can pray in any language. And within this chapter, there are four points.
1. One who prays [the Amidah] needs to have the proper intent for all the blessings, but if he is not able to concentrate on all of them, he should at least concentrate on the blessing of the forefathers (the first blessing of the Amidah aka Shemonah Esrei). If he didn't have the proper intent while saying the blessing of the forefathers, even if he has the right intent for the rest [of the blessings], he needs to return [to the beginning of the Amidah/Shemonah Esrei] and pray. RAMA's gloss: Nowadays we do not return [to repeat the prayer] in a case where one didn't pray with intent, because there's a fair chance that even when he returns [to the beginning of Shemonah Esrei] he will not have the right intent; if so, why should he return (Tur)? ‏And one should not solely pray in his heart, but rather one must enunciate the words with his lips, and make it heard of his own ears but not make it heard in general

2 ב

ולא יתפלל בלבו לבד אלא מחתך הדברים בשפתיו ומשמיע לאזניו בלחש ולא ישמיע קולו ואם אינו יכול לכוין בלחש מותר להגביה קולו וה"מ בינו לבין עצמו אבל בצבור אסור דאתי למטרד ציבורא: הגה ואם משמיע קולו בביתו כשמתפלל כדי שילמדו ממנו בני ביתו מות' (טור):

One should not merely think about the words of prayer in one's heart, but he must actually pronounce the words with his lips and cause them to be heard in a whisper tone in his own ears, however others should not hear his voice (during the Amidah). However, if one is unable to concentrate without raising his voice, he is allowed to do so if he is praying privately, however he may not do so during communal prayer, as this could distract the other worshipers. (gloss: If someone is praying aloud in his own home in order to teach his family members how to pray, it is permitted.)

3 ג

י"א שבר"ה וי"ה מותר להשמיע קולו בתפלתו אפי' בציבור: הגה וכן נוהגין ומ"מ יזהרו שלא להגביה קולם יותר מדאי (דרשות מהרי"ו):

Some say that on Rosh Hashanah and Yom Kippur it is permitted to recite the Amidah prayer out loud, even publicly. (gloss: This is the custom, however one should be careful to avoid raising one's voice too much)

4 ד

יכול להתפלל בכל לשון שירצה וה"מ בצבור אבל ביחיד לא יתפלל אלא בלשון הקודש וי"א דה"מ כששואל צרכיו כגון שהתפלל על חולה או על שום צער שיש לו בביתו אבל תפלה הקבועה לצבור אפי' יחיד יכול לאומרה בכל לשון ויש אומרים דאף יחיד כששואל צרכיו יכול לשאול בכל לשון שירצה חוץ מלשון ארמי:

It is possible to pray in any language one desires. This is the rule when praying together with a congregation, however an individual praying alone should only pray in the Holy Language. However, some say that this (requirement to only pray in Hebrew) only applies when one is asking for his own personal needs, such as praying for someone who is ill, or for some painful episode in his household, however when reciting the regular liturgy established for the congregation to recite, that may be recited in any language. Furthermore, some say that even an individual requesting his own personal needs may pray for them in any language he wishes to pray, except for in the Aramaic language.