Recently, non-ASCII characters in tags were enabled (like they have been on other Stacks) and users started to use the proper spelling.
Trying, as a moderator, to "merge" the old abreviations tag to the newly-created abréviations tag (which is the proper French orthography for this tag) resulted in… problems.
After this "abreviations to abréviations" merge operation, only abréviations appears in tag lists, with the correct total number of questions. Which is fine. But some (not all) questions are now tagged abrviations, that is, the non-ASCII character has been mangled. The tag is displayed like this in questions and questions lists and possesses its own URI. In other cases¹, I'm positive that some of the tags were renamed properly (only a few, which tend to belong to recently created questions) and others wrongly.
Recently the situation changed… and we now have split tags abr and viations. And tag lists are broken as well. Eager to see what will come next…
The tag-merge operation should be fixed. It ought to handle non-ASCII tags correctly.
The FR.SE database needs to be fixed. After those mangled tags are restituted their missing characters (using the fixed tag-merge operation?), tag counts may need to be recomputed. According to my experiment, tag counts were wrong after I manually corrected one tag.
1. Some of the tags I played with before realizing the issue: negation → négation, abreviations → abréviations, etymologie → étymologie, television → télévision.