This class manages the url /home and has a method to identify the language to redirect the application, that ignores the Spring Locale Resolver.

So any configuration done in the Spring Locale Resolver to setup a different language as default is ignored (https://github.com/geonetwork/core-geonetwork/blob/develop/web/src/main/webapp/WEB-INF/spring-servlet.xml#L48-L51)

For LocaleRedirects, check how the language is calculated:

See https://github.com/geonetwork/core-geonetwork/blob/develop/core/src/main/java/org/fao/geonet/web/LocaleRedirects.java#L165

I guess should rely in the Spring Locale Resolver stuff only.