To change the app language, please set the language settings from Settings>App settings>Localization>Language. 

The above setting will change the language for all team members and clients. 

To change the app language only for your personal account, you can change from the topbar.

Note: If you have any personal language selected, the global language settings will not be applicable for you. You'll always see your own selected language. So, whenever you change the localization settings, please keep it mind. 

To customize the language, please follow this doc.