It might be better for you to communicate that you will only be free occasionally in advance. Some very serious language learners may depend on you to be there and if you are not they will likely move on. It is not rude, per se, but it is better to be upfront.

People aren't always sure about what you are up to, inactive, active but invisible, who knows. If it text someone a few times and they don't answer, in most cases I assume they are ghosting me and move on. Or, they might not be interested in speaking to someone every X months. Like Nickie suggests, if it bothers you, you can give people you want to keep around a heads up, maybe as your status?