I noticed that when I create a lingQ in a Japanese lesson (in this case, the first part of the story “I am a cat”) that the highlighting doesn’t work. It only works if you highlight a sentence. I believe it could be because it is expecting whole words (it works fine in Spanish for example, but there the words have spaces between them), but there are no spaces in the Japanese text in this case so it doesn’t work correctly. I know it’s beta, so maybe this feedback will help you fix it.
Yes, the highlighting issue is indeed related to the lack of visible word boundaries in Asian languages. That is essentially why Chinese and Japanese are still Beta. We do hope to resolve those issues in the next month or so. Please be patient in the meantime. There are also some texts with spaces in them which will work better for you.