The word "can" is used in different ways in English—it can mean ability, possibility and permission. When native English speakers talk about what they can do, what do they mean?
A: Can you speak Japanese?
B: Sure, a little. I can order a beer.
A: I mean, can you actually have a conversation in Japanese?
B: No, probably not.
This confusion happened because "can" literally refers to a minimum ability. It does not mean the person is good at speaking Japanese, it just means that their Japanese ability is greater than nothing.
One useful way of thinking about this is using the picture of coffee above. Is there coffee in the cup? Yes, but very little. Answering a "can" question may be the same. Can speaker B speak Japanese? Yes, but very little. Is speaker B good at speaking Japanese? No. Certainly not.
This sometimes causes problems. In Japanese, for instance, "can" is often translated as できる (dekiru). But できる is not a minimum ability, rather it shows some confidence. The result is that some Japanese people say, "I can't speak English." But that sounds very strange in English because it's said in English. Even if you can only order at a restaurant, then yes, you can speak a little English.
However, the gap between English and Japanese is not only linguistic, it is also cultural. Talking about one's ability in Japanese is generally seen negatively. Japanese people will be humble, while in English, it's often ok to talk honestly about your ability.
What should you say?
Sometimes, English speakers understand the underlying question to be, "Are you good at something?" Often, the listener will understand that when you say, "Can you speak Japanese?" what you really mean is, "Are you good at speaking Japanese?"
A: Can you speak French?
B: Well, no, I'm not good at it. I can say bonjour and s'il vous plaît, but that's it.
Non-native speakers can use this pattern too. When asked about skills—even if the question uses "can" as the example above—you can answer using good at or not good at.
So before you tell someone, "I can't speak English," in English, consider telling them, "I am not that good at English," or even better, "I can speak English, but I wish I was better!" Because if you can say that, you can speak English.
literally [adverb]—used to show that a statement is true and accurate.
linguistic [adjective]—having to do with language.
humble [adjective]—showing a modest or low estimate of one's importance or status.
underlying question [noun]—the question the person is really asking, which may be different from the direct question.