Maya, you can ask at local universities in your town whether they have a place, like in their library, where you could put up a notification that you're looking for someone to practice English with. Universities usually have foreign students, who may be interested. My city library in town does something like this. Then you could also look for any language schools in your area for foreigners and ask if you could put up a notification there as well.

By the way, I really don't think that the sentence "I'm not looking for a romantic relationship" is offensive. It's direct, yes, but it's perfectly acceptable. Some may wonder why there is a need to even write that sentence, but most will understand. And don't worry about those who don't understand.

Hi again

I agree with Melanie's comments (and Mustafa's). You could also try looking for any clubs or societies that meet for conversation in other languages. Many towns and cities in the UK have these sort of informal/semi-formal groups that are not classes, just people who want to use their language skills.  I'm not sure if that sort of thing exists in Japan.

I also agree that "I'm not looking for a romantic relationship" is perfectly clear, and not offensive in my culture. You shouldn't have to say this sort of thing, as most men and women on this forum who ask for language exchange want exactly that, but sadly there are a number of people (mostly men) who perhaps need you to spell out precisely what you do want so as to avoid any confusion.

I wish you the best of luck.

Chris