<ナAdjectives>
わたしはマンガが好きです。
わたしは勉強がきらいです
I love comics./I hate studying.
This 好き(です) is ナadjective; It is not a verb. If you use these adjectives, you have no choice but to use this structure. ～は～がすきです。
ルフィーは海が好きだ
さくらは夏がきらいです。

<verbs> 
わたしは日本語が話せます。
わたしは日本語を話せます。
These are a little bit tricky.

I think Japanese people often use わたしは日本語が話せます。
The particle が can add an extra nuance. if you use が, it can highlight the word before が.
So, If you say わたしは日本語が話せます。It depend on the context though, that nuance can be like this; I want to let you know I can speak Japanese.

On the other hand, if you say わたしは日本語を話せます。It simply means I can speak Japanese. You can add additional information like below. In this case, there is no word highlighted, and the speaker deals with these three languages equally.
わたしは日本語を話せます。英語とロシア語も話せます。
I can speak Japanese, English, and Russian.