What changes, in these phrases? The change is the し for the せ. A friend told me that the second one was better for saying I speak little japanese.
Just copied & pasted from my half-year-old answer (though the question itself isn't a duplicate):
Though I translated 日本語を話す into "speak Japanese", the verb doesn't have "be able to speak" sense, so every time you have to explicitly use potential form when you question about ability.
日本語が話せますか？ Do you speak Japanese?
日本語を話しますか？ Are you going to speak Japanese?
The second one is the potential form of the verb. I can speak a little Japanese. Although when using the potential, が is usually preferred over を.
Here are some references on the potential form.