Selections are made on fields in your data model.
If your calculated dimension is rather static, I would suggest to create a new field in your data model that filter, e.g. Customers in Germany. Then use this new field in your dimensions and filters.
If you don't want to do this and if you want to show the field you are selecting in instead of the expression, I think you can use something like (don't know your exact calculated dimension, thus showing just something I made up):
=Aggr( If(Mid(Customer,1,2)='DE',Customer), Customer)