Why change your interface language?
You can use the forum in many languages. Changing your interface language will change the language of the menus, and allow you to use the translation tool to translate posts from other users into the language you set as your interface language from another language. You must have your interface language set so the forum knows what language to translate your post to.
To set set your interface language:
Click on your profile picture in the upper right, then click on the person icon to bring up your user preferences.
Scroll down and on the left side click on interface
Click on interface language and select your language of use.
That’s it!, you should now be setup in your preferred language, and ready to translate posts.