The Laws of Prayer (The morning services) (41)

1. The ideal way of praying is using the Holy Tongue, both in public and in private. (Shulchan Aruch and Mishne Berurah 101)

2. A quorum have the right to occasionally pray in any language as Hashem Himself accepts their prayers. However, an individual praying must pray in the holy tongue as it is the ministering angels who bring his prayer forward and thy only do so if the prayer is said in the holy tongue. Some say that a text of prayer which is fixed for the public may be said even by the individual in any tongue. Others say that even an individual may pray in any tongue apart from Aramaic. (ibid)