I have a contact form in different languages. All Forms (1 per language) store their data on a certain page. When I click "Mails" and that storage page, I see a list of all messages from all forms - so far so good.
But when I click CSV Export, I only get the data from one language. After a while of debugging I think I have found the reason - this is because the export service collects the fields from the first message and based on that field list it builds all the other rows. As other messages are from different forms (i.e. languages) there are no corresponding fields for those messages and they are not exported.
Have a look at https://github.com/Intera/typo3-extension-powermail/blob/master/Classes/Domain/Service/ExportService.php --> see function getDefaultFieldListFromFirstMail
Now I am writing to SO instead of Github Bugs, because I still think that there is something wrong with my setup, as Powermail is quite widely used and such an error would have been reported earlier.