From a certain perspective nothing is impossible, you can code everything.
Impossible in this case means that it’s not in our hand. We can’t change or provide code for all 3rd party apps. NethServer relies on upstream.
As regards Nextcloud it is impossible as I already told you:
For Webtop you need to edit the Webtop database to change language automatically, for Sogo there’s a config file but setting language does not work. In Sogo you have the possibility to choose the language at login. More and more apps just use the browser language. So it’s impossible for some apps and much effort for the others.
Here are 3 explanations:
Please believe it 
As said, I like the idea but it’s really hard to impossible to achieve it.
I understand your concern but I think nowadays it’s expected that users can change the language themselves in their apps profile settings. At work I use mattermost and dokuwiki for providing documentation to the users how to configure their apps/desktop etc.
Which small business server does it better?