There are several inconsistencies in the moderator tools to create and merge tag synonyms. (Note: the tools described in this page are only accessible to ♦ moderators.)
Clicking the “merge” button on a tag brings me to
https://french.stackexchange.com/admin/merge-tags?removetag=TAG_NAME, which shows a form like this:
Old tag, then new tag. Ok.
First inconsistency: the “Synonyms” tab header at the top brings me to the tag dashboard. Fine — but from the tag dashboard, there's no way to get the merge form back (there's a “merge” header tab on the tag dashboard, but it does something different).
Second inconsistency: on the tag dashboard, there's a link “Create a Tag Synonym” which turns up a different form:
Second inconsistency: the two tag names are now called “source” and “target” instead of “old” and “new”. And the help text uses “original” instead of “source”. All these synonyms in the synonym UI make things a bit confusing.
Third and worst inconsistency: the two tag synonym interfaces put the source and target (or old and new) tags in the opposite order. I prefer old then new, but what I really want is consistency.