Passion.io was designed in English and the in-app buttons cannot be changed to another language at this time. However, you can choose what language you use within your PassionProduct.
To create a Product in another language, go to PassionProducts > Click on the New Product button Or Clone Product> and fill out the content fields in your chosen language.
Your apps navigation buttons will remain in English, but the content of your product will be in the language of your choice.
If you are targetting more than an audience, and want to have English + Another Language, please watch the following video for reference: You can learn more here