Answer: The Mishna (Sotah 32a) teaches that
one may recite the shema and the amida in any
language. Thus, Rambam (Kerias Shema 2:10) and the Shulchan Aruch
(OC 62:2) write that one can recite the shema in
any language providing they pronounce the words clearly. The Shulchan Aruch (OC
101:4) writes that one may daven the amida in any language though
brings different opinions as to whether this applies equally to public and
private prayers.
Tosafos (Sotah 32a) writes that one who does not understand
what they are saying when they are reciting the shema or davening
has not fulfilled their obligation. They should rather recite it in a different
language that they do understand. The Magen Avraham (62:1; 101:5) paskens
like Tosafos (see Pri Megadim, Eshel Avraham 62:1).
Nonetheless, the Mishna Berura (101:13) quotes the Chasam
Sofer (OC 84; 86) who demonstrates that one may only daven in a foreign
language as a temporary measure. Elsewhere (62:3) he explains that as there are
certain words that cannot properly be translated, such as veshinantam and
totafos in the shema, one should stick to lashon hakodesh
as much as possible (see Biur Halacha 62:2; 101:4). The Aruch Hashulchan (OC
62:4; 101:9; 185:3) adds that even the names of Hashem cannot properly be
translated, and one must not therefore, daven in a foreign language.
Nonetheless, R’ Moshe Feinstein (Igros Moshe OC 4:70:4) writes that
while one is learning Hebrew, one may daven in English, though they must
ensure to only use a good translation (see Rivevos Ephraim 3:92; 4:44:34).
In conclusion, it is certainly preferable to daven
in the original lashon hakodesh even if one does not understand the
words. Ideally, one should use a siddur with translation so that they
can understand what they are saying. Only if one cannot read Hebrew, should
they recite the tefillos in English.