When you translate a Blocks app, it is important to understand that Wix sites has two types of translations:
For example, a developer might want to have German as their account (Editor) language, but the site they are developing is intended for Spanish speakers, so the site language will be Spanish.
The first step for translating your app is selecting the languages you'd like to be available.
This adds the language to the language picker in your top menu.
The second step, is translating user-interface elements, such as button, texts and so on.
Now, when your app is installed on a site that's translated to that language (for example, Spanish), these texts will appear in Spanish as well.
Translate a custom panel To translate custom panel texts, go to the Panels tab and follow the same procedure as translating your site elements.
To preview your translation in Preview mode in Blocks: