There are three language settings to adjust.
One of them is the interface language which is at the top of the settings page. That changes the language used on the interface itself.
The other two are buried a bit deeper, if you go to Preferences > Other (on the desktop web at the left of the screen, on the mobile web at ☰
In the Other section, choose English from the "posting language" menu, then tick the English box in the "filter languages" section. Then click "save changes".