I know this isn't what you asked for, but I think the best way to go about writing songs is to write them in your native language, or any language that you're comfortably fluent in. If you want to write in other languages, you'll get better results by passing complete creative freedom to the writer and making the project into a collab rather than being directed by only one person. The reason I believe this is because using your most fluent language gives you the ability to express yourself as creatively and as poetically as you want, which is important for lyrics (a form of poetry!) You're also more likely to be aware of nuances that language-learners aren't. For example, with English, syllable stress is fairly important for a natural-sounding melody, and mistimed stress is a common feature in lyrics written by English learners.
I wouldn't be able to do any song justice by trying to write it in Japanese.
Then write it in English and do it full justice! There's plenty of English vocalsynths available now, and even if the specific voice you want to know doesn't have English capabilities available, you can certainly make do with a Japanese library.
At one point I had wanted to learn Japanese in order to write lyrics, but once I let myself write in my native language, I could focus so much more time on music production and on tuning the vocals than on studying grammar. In the end, I was more satisfied with the flow of the songs, and not terrified to publish in fear that I wrote something wrong. (I'm still studying Japanese, but my goals have shifted towards working in Japan.)