For starters speaking any other language within a professional environment is unprofessional.
Have you ever worked in an organisation that boasts of graduate employees only for your co-staff to be speaking yoruba or igbo indiscriminately only correcting themselves when internal control is around?

Here are few reasons why you should stop it today.

-It reduces the organisation's brand.
-It reduces the value of your services especially of your services are targeted at high networth clients.
-It creates diversity within the office environment.
-Ethic barriers are created leading to hostile office environment and reducing interaction among staff.
-While it helps clients feel at home, over-indulgemce with clients should be avoided.
-You're a graduate, your should sound like one and be able to express yourself fluently in English.

Of course occasional speaking in local dialect is inevitable especially when there's a hot gist to serve, but it should be curtailed and reduced.