Neither Forms nor Workflow installs properly on a non english server.
Specifically, my server has SharePoint installed only with the French language (1036).
When I ran the setup, it failed with this error:
Impossible de trouver une partie du chemin d'accès 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\HCCab\1033\Nintex.Forms.HC.cab'.
This is due to the installer that try to write files in
"C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\HCCab\1033"
But the 1033 folder does not exists.
It can be worked around easily by creating manually this folder.