Hello,
I have the same issue with TranslatePress and WPS Hide Login.
It doesn’t work even with “Slug for the main language” deactivated.
The custom WPSHideLogin page appears correctly, but when you login with correct credentials an error of unreachable page comes out (I noticed that URL change without “/” between site name and custom login page – e.g.: https://www.testsite.comtestpage instead of https://www.testsite.com/testpage/
NOTE that by disabling TranslatePress Plugin everything works fine!
A solution for this issue would be very appreciated because I found your plugin very useful.
Thanks and best regards.