The Windows input method adapter currently assumes that Windows provides input method support for the Java default locale (in WInputMethodDescriptor.getAvailableLocales) or even all locales (in WInputMethod.setLocale). This is wrong. The Java default locale is a very poor indicator for the Windows input method support - it starts out as the current Windows locale, which may or may not match the locale of the input method, and can be changed by a Java application at any time.

Instead, the host input method adapter should try and find out which is the primary locale supported by Windows and return that from WInputMethodDescriptor.getAvailableLocales, and also return true from WInputMethod.setLocale for other locales that can be supported (such as English and other Western European languages).