The only chance you get to set the language is when you create the app. I have not found a way to change the language. If you set the properties on the object at the top of the caller apps tree to allow selection of the language it allows you to add languages when you edit properties. That is for multiple language in one app.
So I created a new app, called it Dutch and selected Dutch language, clicked ok. Right clicked on it and selected properties, clicked on termination in the general tab and select dial by name. Click on the languages tab and it says it is editing Dutch. Click ok then save and deploy. Refresh the list of deployed apps, click on associations, right click and select new assocation. Name it Dutch and select the Dutch caller app. repeat the association id in the Called number field (100832 this time)and click ok.
If you dial 100832 now you get a wave off. I dup the station of the working caller app and create 100832. Then I dial it and it says "Spell the last name first" in US english.