Beur Halachah to Shulchan Aruch, Orach Chayim 101:3
Magen Avraham wrote in the name of Sepher Chasidim that it is preferable to pray in a language that one understands if one does not understand the Holy Tongue. It is clear from Sepher Chasidim 588 that this is specifically if one is G-d fearing and one’s desire in so doing is only in order to pray with proper intent. Otherwise one should pray in the Holy Tongue (Hebrew). The reason for this is that the Holy Tongue has much deeper meaning (סגולות רבות) than any other language. It is the language in which the Holy One Blessed is He speaks with His prophets… Our Sages of blessed memory said: “With the Holy Language was the world created”… Furthermore, when the Great Assembly redacted the text of the [set] prayer, they were 120 elders and among them a number of prophets. They imbued the words and letter combinations of each blessing with many deep and lofty secret [meanings]. When we say these words in the language [chosen by] the Great Assembly, even if we do not understand, nevertheless, our prayer is rightly effective, for the words themselves effect their holiness above, unlike when one prays in a foreign tongue.