Translation:Spanish is a popular language.
It's a common way to talk about languages - using the neuter plural form of the adjective.
Τα ισπανικά, τα ελληνικά, τα αγγλικά, ...
I don't know why, but one guess is that this may be connected to the fact that the neuter plural form of adjectives is also often used to form adverbs, so you get sentences such as Μιλάω αγγλικά "I speak English-ly" and that adverb got reinterpreted as a neuter plural noun.
Any reason why it is not δημοφιλος for the masculine?
It's a different kind of adjective inflection -- diptote (two kinds of endings: masculine/feminine are the same in -ης and the neuter is different in -ες) rather than more common triptote adjectives with three kinds of endings (-ος, -η, -ο or -ων, -ουσα, -ον).