Force specific language for user activation mail and edit localized email templates
When using a frontend form to create a new user it is actually straight forward, because you can easily send a preferredLanguage attribute with the request that sets the language on creation (see this line in the user controller). However, if you want to use the CP form to create a user, there doesn't seem to be an out of the box solution for your problem. Here is what you could try:
There is a template hook in the user edit template. You could use this to inject an input that sets the preferred language. You could even show that input for new users only. In the simplest case it could look something like this:
&lt;input type=&quot;text&quot; name=&quot;preferredLanguage&quot; value=&quot;de&quot;&gt;

You surely would want to have a select or something with the available languages. To inject the markup into that template you would have to create a module or plugin and then follow the docs for template hooks.
PS: If you wonder where the user language is stored, it has an extra table called userpreferences storing stuff like this in JSON format.