Makapagtagalog is the appropriate form to express the ability to speak Tagalog.
By contrast, the maka- prefix when attached to a noun would denote a predilection or liking for that noun. Thus, maka-Tagalog would mean a predilection for or a preference towards things pertaining to Tagalog culture in general (Tagalog food, art, dance, language, society, etc.). Other examples are maka-Amerikano, maka-Diyos, maka-tao, all denoting particular connections with the aforementioned nouns.