When the manual in your current language is not installed, yet other
manuals are available, the help dialog will now propose either to read
the manual online (as was already the case) or to select a manual in
another language instead. This is even more important since we don't
have as many manuals as localizations of GIMP. Therefore if one took the
explicit step to install a manual in another language, it makes sense
that means one may know the alternative language. As an example, we have
3 Chinese localizations (zh_CN|TW|HK) but only a zh_CN manual. I could
definitely imagine someone with a zh_HK GUI to go for the zh_CN manual
as a fallback. Or other languages even, whatever!

This is a first step. Right now once one chose the alternative help
language, it is not possible to reset it yet (except by editing
"help-locales" in gimprc). The next step will be to add the settings in
Help System preferences.