Hello,

The default language of your site is configured to be ΕΛΛΗΝΙΚΑ So you need to insert messages in this language and then translate them with WPML.

Your registration text is in English and should be in ΕΛΛΗΝΙΚΑ http://prntscr.com/juye5b then find this sting and translate http://prntscr.com/jv0ve1

As for Register text, exchange the translations.

In result, your translation will be displayed correctly.

Best Regards