Tosefta do Powtórzonego Prawa 6:6

The blessing of the Hallel, the Shema, and the tefilah (i.e., the Amidah) may be recited in any language (cf. Sot. 7:1). Rebbi says, I say that the Shema may only be recited in the Holy tongue, as it is said (Deut. 6:6), "And these words, etc."