Oh, now I got it. It is really multilingual. Only if the option is deactivated is it still stored in the array.
Photo title and description are always saved with the language abbreviation currently set. So, if multilingual disable, and if I change the language more often, then many language abbreviations are in the database. If I save a photo description that is already stored in en_US now with the setting de_DE, then no additional language will be added, but the language code will be changed. The display of the description is independent of it.
So with multilingual disable it makes no sense.
There is a fallback to en_US if there are multiple languages. But if there is only one, no matter which, including others, then it will be displayed. A question that I am no longer really interested in, what happens if zenphoto is set to en_US and the photo description is available in German, French and Italian (without English). Which description will be displayed?
OK, on the whole it's a bit difficult to understand.
Thanks for the information.