To put the Monaca Localkit UI into English

On a Mac, go to the command line

LOCALE=open-a "Monaca Localkit"

If you launch it in , it will be in English (if the local kit is in the application folder).
Or

LOCALE=open-a~/path/Monaca\Localkit.app

You can also specify a path as shown in (if the local kit is in the ~/path folder).

Even in Windows, if you set LOCALE to en in the environment variable, it becomes English.
(Ja for Japanese)