If you want to translate the app, You are using a single language and not using wpml plugin.
Unzip build-app-online.zip folder and open build-app-online/languages/en.php file.
Translate all right side words/sentences save and zip the build-app-online folder and upload the plugin
If you are using WPML plugin or Polylang plugin.
Unzip build-app-online.zip folder and duplicate the en.php file in build-app-online/languages and rename to your language code. Example if you are using Arabic creates a file ar.php and so.
Open duplicated files Translate all right side words/sentences. save and zip the build-app-online folder and upload the plugin