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Apparently he did not understand the documentation linked by a template in "/qqq" pages, and tried to translate the message-ids (in English) to Oromo [om], instead of translating the messages themselves; this is reversed. This looks like an error, not intended vandlism. But I wonder how to make this less confusing (we need to cite the message-ids in the related templates). May be we could add a top banner when we follow the link "Modify this list" shown by the "Related" template; that banner would explain that this is not intended to make any translation, just to link messages in the documentation pages and showing their current translation. To actually translate messages, users need to follow the links displayed on the rendered list, not modify the list itself.
Note that this bad edit is quite old (on 7 March 2021) when that user was still new on the project (he joined and created his user page on 12 February 2021), and may be there was still not a good enough documentation and navigation or UI on the site was more confusive. It's strange that it was not detected before (several other incorrect edits of "/qqq" page at that time were already reverted quite fast in March 2021). After these few incorrect edits, this did not occur again later.