I guess each country is different. In the USA, professional terps are supposed to follow guidelines of appearance and behavior. The reason for dressing a certain way has nothing to do with style or personal preference. One reason is to make it easier for the Deaf client to see the signs clearly, and not get tired eyes. Another reason is to look professional, not like a volunteer terp from off the street. A third reason is to not embarress the Deaf clients in front of the hearing clients.
If a terp has a professional attitude, then he/she should not mind dressing appropriately.
If a client complains about the terp's appearance or behavior, nothing bad happens to the client. They don't "lose" terps. If the terp wants to keep working, the terp must make the changes.
Of course, a terp should never, never criticize the clothing of a client.
I am curious. How do you hire terps in Bavaria? Thru an agency, or as individuals in private practice, or ...? Who pays?