If you want to change the language of the contact form element of Quix, from the language override panel, you have to first input the language constant name and the correspondent text you want to show in your preferred language.
Extensions > Languages > Overrides.
In the filter drop-down menu, you have to select the language.
Now click new, copy & paste below language constant in the Language constant field, in the text field you have to write text which you need to show your site front-end.
COM_QUIX_EMAIL_SENT_SUCCESSFUL COM_QUIX_EMAIL_SENT_ERROR QX_CF_YOUR_NAME QX_CF_YOUR_EMAIL QX_CF_YOUR_SUBJECT QX_CF_YOUR_MESSAGE QX_CF_SEND QX_CF_YOUR_NAME_HINT QX_CF_YOUR_EMAIL_HINT QX_CF_YOUR_SUBJECT_HINT QX_CF_YOUR_MESSAGE_HINT
Hopefully, you can easily change the language of the element.