I just installed simplewall and it's displaying everything in Japanese. I have the Japanese language pack installed but it's not the default language. My Japanese isn't nearly good enough to figure out how to change the language.
Here's a screenshot showing the application as it appeared after running for the first time and my language settings. I am using Windows 10 Pro with latest updates.

I got around the issue for myself by deleting Japanese from simplewall.lng.
Maybe this is bug.

You can change language in menu. Language submenu always displays in international locale or in settings (press F2) on 1st page.
Most helpful comment
Maybe this is bug.
You can change language in menu. Language submenu always displays in international locale or in settings (press F2) on 1st page.