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Problem:
Would you please help us in translating the interactive forms with Formidable PRO addon? We have tried already below solutions, none of them worked.

Solution:
It seems that the glue plugin between WPML and Formidable was not made to have the placeholder string available for translation.

Now, can you go to wp-content/plugins/formidable-wpml/controllers folder, edit the FrmWpmlAppController.php file, and make a change on line 606, please?
Basically, just change the code from this:

$keys = array(
                    'name', 'description', 'default_value',
                    'required_indicator', 'invalid', 'blank',
                    'unique_msg',
                );

to this:

$keys = array(
                    'name', 'description', 'default_value',
                    'required_indicator', 'invalid', 'blank',
                    'unique_msg', 'placeholder',
                );

As you can see, now the placeholder is added to that array so it can be made available for translation.

Relevant Documentation:
https://wpml.org/documentation/plugins-compatibility/how-to-translate-formidable-forms-with-wpml/
https://wpml.org/forums/topic/formidable-placeholder-text/