Normally, I would say choose the interface language that you prefer because the content won't change, but this is not true for CiNii. If you choose to use the English language interface, then any articles that have an English language version of the title and abstract will display. If you click on the Japanese button, then the titles of the articles will display in the language they are written in. See these two examples:
This is the English language interface. The first article has an English title.
After clicking on the Japanese button at the top right of the screen, the title of the first article changes to Japanese.