After login - just look at the top right. You'll see a language selector there - with a flag. Just change your language right here. If you're logged in, we'll remember it's you and this is your language choice. Note that this choice will impact what you see on the web, as well as emails and notifications.
Pop over to Account Settings, then scroll down. You'll see an entry called 'Language'. Expand this section, and cllick on the Change button.
Next you'll see the languages that we support ( there's plenty there already, with more coming! ) - find the one you want, and touch select. And that's it!