Hi,
Please check your language file.
The file is maybe missing, not accessible or could contains wrong data (so Joomla can't parse it correctly).
About the 500 error, Joomla says that the layout is missing.
Problem is, we don't have more information. If you activate the Joomla debug mode, it will be possible to have more details.
I think that you have modified your checkout workflow with a "step" which does not exists.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.