is reading tehillim everyday in English just as effective as Hebrew?
Speaking from the heart is the main point, for our direct connection with G-d, should be without intermediaries, like a child speaking to his Father. But if one can understand and speak to Him in His Holy Language, especially if you can truly identify from your heart with the holiest of the wordings of the most choice prayers and hymns ever written by the prophets and sages over 3300 years, you gain that much more connection with Him. Also, in the original, if you understand it, there's a certain power in using the words of King David, and it's easier to identify and connect with him and his original feelings! Hebrew has such deep kabbalistic meanings in every letter, root and word, that God actually created the world with the Hebrew language (the verb used in creation is "and He said", i.e. in Hebrew, and the name Adam, for example, only makes sense in Hebrew, for he was taken from the “adama”= ground). But in the meantime, until you eventually learn Hebrew, it’s definitely beneficial to say Tehilim in English, just make sure that it’s an orthodox Jewish and understandable translation.