Hi,
What are the culture settings set for your site and for the users (e.g. public user)?
What are the culture settings in their browsers?
Do they have user interface tab opened within the same browser?
What is the exact version of
CMS you are using? In 5.x versions were fixed some bugs regarding cultures - could you please apply the latest hotfix for your version or upgrade to version 6?
Best regards,
Juraj Ondrus