Localization Correspondence Management Data Dictionary fails

Hi,

we followed the instructions on

https://experienceleague.adobe.com/docs/experience-manager-65/forms/letters-correspondences/data-dictionary.html?lang=en#localize-data-dictionary

regarding how add localised values for data dictionary elements.

We fully made it through the whole process nearly successfully. We added locale for French and English and we were able to test this: We downloaded again localisation data and it contains the additional languages in the proper format.

But when it comes to the point: Change the browsers locale, we are failing. We removed every language except french, and played around with the different variances of fr_FR, fr only. None of them works, neither for French, nor for any combination of en_EN, en_US, en_GB and so on. 

So what is that trick to make use of the translated data dictionary?