Merge dialog shows continuing...but no error is given to the user when trying to merge lemma that do not match exactly.
Try to merge these 2 Lexemes:
eye to eye https://www.wikidata.org/wiki/Lexeme:L190266
No errors are displayed on the UX merge dialog.
No JS errors found in Firefox browser console.
On the mailing list, Nicolas Vigneron responded that:
for a given language the lemma has to exactly match (because otherwise, you would have two lemma with the same languages which is not possible).
So before merging, you can either change the lemma to be the same or change the language of one of the lemma (en-gb ?).
It would be useful to give the user a simple warning or error message such as the following:
Lemma do not match exactly, so merging is not possible.
For details and workarounds see <some wiki page URL that explains more and mentions Nicolas' workarounds>