You can change the display language to Japanese or English by following the steps below.
Web version:
1. Tap the Earth icon in the top right.
2. Select a language.
App version:
1. Tap Profile on the bottom menu bar.
2. Tap the gear icon in the top right.
3. Tap Settings > App Language.
4. Select a language.
Please check here for how to change the language of posts and comments.