Bank Account in Bahrain mandatory for expats?

Well, yes and no.

There is no requirement to open a bank account for any expat who is here on a residence visa.  That is entirely your choice.  And if you have an agreement with your employer, your salary can be transferred offshore.  In practice, they don't generally do that except for very senior roles - because of costs and complexity that is involved in that.

However, there is a recent initiative of a WPS (Wage protection system) that is planned to be rolled out this and next year, by the Government.  This will make it mandatory for the employer to transfer an employee's salary into their bank account.   By salary, I mean, the amount that is listed on your offer with LMRA and is subject to GOSI deduction with SIO.   Any amount above that is free to be transferred anywhere - but again subject to agreement with your employer.

Thank you very much for your information. Any idea where I can get official documents about that?
Thanks again

Official documents for what?

Wps announcement is in the newspapers and when implemented will be on lmra website.