Popup required field

At my website when I fill in the required field and I 'forget' to fill in 1 or more field, I see a popup with only the name of the website.
Can I fill in some text like 'This field is required' in this popup?

Hope you can help me.

Hello Ingrid and thank you for posting.

I suspect that you are using Rapid Contact Ex in a language other than English, so you have to copy (and translate if possible) the following two language keys into your respective language .ini file:
PLEASE_FILL_IN_ALL_REQUIRED_DETAILS="Please fill in all required details"
FIELD_S_IS_REQUIRED="Field '%s' is required."

The second language key is not available in your version, yet. I will release it later today, so please make sure to update, so that you get a specific error for the required fields instead of the generic one, which is now only used as a fallback.
Thanks again!

Thank you very much it is working now.
I only can't find in which .ini file I can translate it to Dutch?

Hi again Ingrid.
You should look for language/nl-NL/nl-NL.com_rapidcontactex.ini and language/nl-NL/nl-NL.mod_rapid_contact_ex.ini.
If these files do not exist, you can copy the en-GB files from the en-GB directory, and rename them to nl-NL.

Let me know if you need any further assistance!
More help is available online:
https://mavxr.com/help-base/33-changing-texts-in-joomla-extensions
https://demo.mavxr.com/translation-using-language-ini-files