I can confirm the same behavior in french.
$sudo gdebi mysql-workbench-oss-5.2.21-1ubu1004-amd64.deb ...snip... Voulez-vous installer le paquet logiciel ? [o/N] :o
--> won't work (with any character)
LC_ALL=C sudo gdebi mysql-workbench-oss-5.2.21-1ubu1004-amd64.deb ...snip... Do you want to install the software package? [y/N]:y
--> works
I can confirm the same behavior in french.
$sudo gdebi mysql-workbench -oss-5. 2.21-1ubu1004- amd64.deb
...snip...
Voulez-vous installer le paquet logiciel ? [o/N] :o
--> won't work (with any character)
LC_ALL=C sudo gdebi mysql-workbench -oss-5. 2.21-1ubu1004- amd64.deb
...snip...
Do you want to install the software package? [y/N]:y
--> works