The language of a song is the official language in which the lyrics are written and sung. The vast majority of the songs on Rock Band are in English, however there is a notable number of Spanish songs as well, alongside a very small number of songs in other languages including French and German.
In gameplay, the language in which a song is set has no effect on any aspect of gameplay. Since vocalists are not required to sing lyrics accurately, simply mumbling unfamiliar words will have the same effect as singing regularly in English.
Some notable songs that are in a language other than English include "Me Enamora" by Juanes (Spanish), "Oye Mi Amor" by Maná (Spanish), and "Du Hast" by Rammstein (German), all three of which are featured on the Rock Band 3 on-disc setlist.