You can set Mastodon's web interface to be in another language if you want:
1. Log in through your server’s website or the web app
2. Click ️ Preferences
3. Select the language you want from the dropdown "Interface Language" menu
4. Click "Save Changes" button
You don't have to use your server's default language, all the languages are available on all the servers.
This only works when you're signed in though, so you will still see your server's language on the sign-in page.