https://discourse.crowdin.com/translate/f3230e7607a36bb0a2f97fd90605a44e/246/en-ru/24?filter=basic&value=0#q=js.topic.merge_topic.radio_label
Looks like there is a bug for 6 months now, the previous version (with 2 upvotes) was correct. Currently it says “There was an error while moving posts to this topic” instead of “Existing topic”.
I don’t know if there is a better way to report such bugs than creating a topic here.