Equal Employment Opportunity Commission guidelines. Even commonplace interactions could have subtle cultural nuances to take into account.
Consider utilizing employee surveys to gauge how employees feel on these topics as a means to discover issues before these issues cause employees to leave. You may need to take a more active role in helping them adjust to the culture at work as well as in their new communities.
The office's extremely tall guy, meanwhile, may feel the exact same way. Keep trying to learn about your staff, especially the new hires. You have to have sensitivity training to try to limit any preconceived ideas from workers concerning other cultures.
For example, avoid scheduling client lunches during a time of fasting or holding meetings during a time of prayer.