[Tarto]

Hello

I agree with KyleGoetz in that you'd better avoid using を if you're not totally sure about what you're doing, even though it is used a lot (I work for a small Japanese company in Nagoya and it's used a lot in emails for instance).

However, I strongly disagree with the idea that は "definitely carries the extra implication of "I can speak Japanese [as opposed to some other language].". In fact, は has several usages, but only one of them is the expression of a contrast. If you only say 日本語は話せます, it does not imply that you cannot speak other languages.

Note that in actual conversation, は is indeed very often used:
- in a friendly, casual context, the particle is often skipped and the following structures are extremely frequent: 日本語、話せる？, 日本語、わかる？ or 日本語、大丈夫？. Note that 話すことができる is not very common
- in a more polite situation, you would use the ます forms of the previous structures, for instance 日本語は話せますか？, 日本語はわかりますか？, 日本語は大丈夫ですか？ etc.
- if the situation requires you to be very polite, you cannot directly use the potential form as it implies some form of evaluation of the skills of the other person. You must drop the potential form and use keigo instead: 日本語は話されますか？. Note that this is more likely to be asked about another language, since using keigo already implies that the other person speaks Japanese well enough to understand it...