The issue you're describing is not actually a problem in the Extension Library,
but in the XPages Runtime. The issue is tracked as SPR#MKEE8HNKWL, and is likely to be fixed in Domino 8.5.3.
The issue is that the XPages runtime has a file "javax/faces/Message_en.properties", but does not have the corresponding Messages.properties (without a locale suffix), so when XPages attempts to access that file in a non-English language, that exception occurs. The file contains some message strings about validation that appear in the Display Errors control.
To workaround the issue, you can either change your application's Localization Options to use English, or you can change your server language to English.
To change your application's Localization Options to English, in the Application Properties, you will need to enable localization, add the language English, set the default language to English, save, and then in the menus do Project, Clean for that application (which will re-build and re-sign all of your XPages). For details on the Localization Options see:
http://www-10.lotus.com/ldd/ddwiki.nsf/dx/UsingLocalizationOptions.htm
To change your server language to English, in MS Windows, go to Control Panel, Regional and Language options, and change the language there.
Thanks for your feedback.