There is a problem with building/installing rpm packages which contains %lang macros in the spec file. The problem is that files in %doc are getting the same language as that set in the last line of %files. So if there are some files labeled with %lang macro at the end of the %files section and the last one is %lang(xx), documentation files will get default language set to xx (no matter what lang it will be). And if there's no xx language in LINGUAS or LANG variable, documentation files won't be installed. This problem is common for all of the 2.9x rpm versions to current (3.0.1).
Fixed in rpm-3.0.1-4 (and cvs). Thanks for finding.
Actually the inheritance of last %lang to %doc is what was fixed. Files that are marked with %lang are not installed unless mentioned in LANG or LINGUAS at install time.