How to determine the merit of an encyclopedia?
This might not be what you want hear, but I think this kind of product doesn't exist anymore in any meaningful way. Essentially you are asking what would be a reputable universal encyclopedia (with a grown up target audience) in different languages. 

I checked a large high quality German book store (Kulturkaufhaus Dussmann) and I didn't find a single one matching your query. There are a number of kids general encyclopedia, for different age groups and of different quality and there are a whole bunch of very specialized encyclopedias, from Star Wars to medical but no universal ones for grown ups. 

20 years ago this would have been a good question and there were multiple options in all major languages that could be compared to each other but this is not the case any more.