The Halacha permits praying the Amidah and reciting Birkat HaMazon in any language. This applies on condition that one understands that language and the translation is accurate. Therefore, technically, one could pray from the variety of English Siddurim that exist today.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) discusses a case in which a person does not understand Hebrew, but prefers to pray from the original text in Hebrew. He rules that Hebrew is analogous to “Type O” blood, so to speak. It’s universal, and therefore, even if someone doesn’t understand what he is saying, he fulfills his obligation.
This is not the position of Rabbi Zalman (R. Shneur Zalman of Liadi, 1745-1812) in his Shulhan Aruch HaRav, who holds that someone praying in Hebrew must understand the words he pronounces. According to him, if he doesn’t understand Hebrew, it would be better to pray in English and understand what he is saying. Although he concedes that the common custom is not like that, nevertheless that was his position.
Accepted Halachic practice is in accordance with the Mishna Berura. Therefore, one may continue to pray in Hebrew, even if he doesn’t understand the words. However, in light of the dissenting opinions, one should devote time, in addition to his other studies, to learn the translation of the original Hebrew text. Especially regarding Birkat HaMazon, which is a Misva M’Doraita (Commanded by the Torah), one should make an extra effort to fulfill it with all of its stringencies. Since according to Rabbi Zalman, one does not fulfill the Misva without understanding the words, he should learn the translation, or at least the basic concepts of each Beracha.
One may pray and recite Birkat HaMazon in Hebrew, even if he does not understand the meaning of the words. However, one should make an effort to learn the meaning of the text.