I checked the MailPoet plugin an it’s source code. I couldn’t find a way to make it work with my plugin.
But what I have recognized is, that the MailPoet plugin does a lot of language overwrites itself. This should not be necessary for any plugin. I would say, that those functions are causing the issues with my plugin.
If you still want to use a language variant like “de_DE_formal” you could do the following:
– Create a folder “wp-content/languages/wysija-newsletters”
– Copy over the file “wp-content/plugins/wysija-newsletters/languages/wysija-newsletters-de_DE.mo”
– Rename it to “wysija-newsletters-de_DE_formal.mo”
I hope this will work for your setup. And I also hope that the MailPoet removes all thos (in my opinion) unnecessary functions causing all those troubles.