A person who is reading and feels a need to translate after each word or pasuk (verse) into a language that he understands is permitted to translate as needed.
Sources and Explanations
Now, regarding translating words in the middle of the Amidah prayer into the language familiar to him in order to have kavanah, we find a dispute among the Acharonim whether it is permitted.
The Gaon Rabbi Moshe Feinstein zt”l, in Igrot Moshe (Orach Chaim vol. 3, siman 8), wrote that it is forbidden to interrupt in order to explain aloud in the vernacular the intent and meaning of the tefillah, even though this helps him direct his heart more toward Heaven. Not only is it forbidden in the middle of the berachot, but even between the sections, and even in Pesukei DeZimrah it is an interruption. Furthermore, by translating he spoils it, in that he adds words out of place.
In Responsa Az Nidberu (vol. 10, siman 20) he disagreed with him and proved that it is permitted. So too wrote the Gaon Rabbi Menashe Klein zt”l in Mishneh Halachot (vol. 5, siman 26), based on what is explained in Responsa HaRashba (vol. 1, siman 474) and in other poskim, that “changing from the formula that the Sages established” refers only to one who changes the essence of the berachah, for example, if he concluded “Baruch Atah Hashem, Who graciously grants His people Israel knowledge,” and the like; but whenever one does not change the essence of the berachah, but rather in the middle of the berachah adds some words, this is not called changing the formula. And know that this is so, for nowadays there are versions upon versions, and there are hardly two siddurim whose texts are identical, and sometimes people pray from this siddur and sometimes from another, and Heaven forbid, no one would open his mouth to say that this falls under changing the formula. Therefore, one who says in the middle of the tefillah “Ribona de’alma” or “heiliger Bashefer” (holy Creator) certainly does not fall under changing the formula, and also regarding interruption it seems that it is not considered adding to the text of the berachah. He also cited in the name of the Gaon Rabbi Shlomo Kluger, in Responsa Tuv Ta’am VaDa’at (Kamma, Yoreh De’ah siman 211), that it is permitted, since one may pray in any language, it is as if he added some words in Lashon HaKodesh, and in such a case it is certainly not an interruption.
Now, all that has been said applies to the berachot of Shema, Pesukei DeZimrah, the Amidah prayer and the like, where there is a concern of interruption since they are said after berachot. But in the middle of reading Tehillim, where one may interrupt for anything, it seems that even the author of Igrot Moshe would agree to be lenient in this, in order to aid his kavanah in the reading.
From the sefer “Shaf Veyativ – Laws of Saying Tehillim” by Rabbi Shay Tahan, Chapter 5: How to Say Tehillim and Precise Pronunciation, Halacha 21 (p. 79 in the print edition). Translated from the Hebrew original.
