I noticed on the store page for Re;Birth that it says it supports Japanese text.

As someone who's been learning Japanese for nearly five years and would like to play this in Japanese for studying reasons, how does one go about switching between English/Japanese text?

My OS language is set to Japanese, but the game text is still in English...

If you are using Galaxy, you can go to configuration and choose your desired language from there (see the attacment), when you do it, it will download the necesary files. You can do that when installing the game through Galaxy too.
If you are using the offline installers i don't know, maybe there are a installer for every available language?

Ah, it's a separate offline installer. I never thought of that.

Thanks.

No problem, nice to hear you sorted it out :D