It was implied by all the excellent posts, but not quite stated for the record, that Tagalog is a cultural group as well as their language - and to some degree, the geographical area, which is around the capital, and hence chosen as the base for the attempted national lingua franca.
The lingua franca in the north in the Cordillera is Ilocano [the language from the coastal north], though… in addition to all the local languages and dialects.