MoinMoin tries to adapt the user interface to the language the user prefers.
If the user puts a specific language preference into his user preferences, that language will be used for the user interface. But the user doesn't even need to do that if he already has configured his browser with his language preferences.
If there is no specific user preferences language setting, moin tries to adapt to the languages the user configured in his browser. So if the browser tells moin that its preference is Canadian English, then German, then English in general and moin has German and English available (but not a specific configuration for Canadian English), then the user will get a German user interface.
If there is no common language in the user's browser configuration and in moin or if you have set language_ignore_browser = True, moin will fall back to using what is configured as language_default. This is also the case if the user's browser does not specify any language.
The usual case when you want to set language_ignore_browser = True is when running a local wiki with no international audience and you maintain the wiki in only one (your local) language. Don't forget to specify your one-and-only language using language_default when doing this.