GNU webmasters proofread the texts before posting them and fix the bugs reported by www.gnu.org visitors, but occasionally some mistakes do slip into the pages. Translators are probably the people who read the pages most carefully, so it’s them who are likely to find those mistakes first of all.
There is a trend for translators to fix the bugs in their translation instead of the original page. This is understandable: you have to contact additional people (webmasters) in order to fix it on the English page, so it’s easier just to make the translation. However, there are reasons why you shouldn’t leave bugs on the English pages: