Login to your account and expand the Box Settings section in the lower right corner. Click the edit icon beside the "Response Page Language" heading and a new window opens where you can select your desired language (currently either Spanish, German, or English by default).
Dashboards and other pages only seen by account owners and box managers will remain in English when language settings are changed. In other words, language settings only apply to the suggestion boxes where your respondents leave feedback.
Language settings are applied to each box so an account owner with multiple boxes can assign a different language to each box.