So this is something that could use a bit more explanation in the website builder. Once you add the languages, use the switcher to change languages. Then when you change something, it will only show up when that language is selected.
So, for example, if you use the selector to select Japanese, you can edit all of the existing text and change it to the translated version. But when you choose another language, it will show what you have changed for that language.
I know that was a pretty bad explanation but I hope it helped.