Change language for billing with Braintree

Hi,

I've some difficulties in changing the language of the Braintree Dropin. I changed the "locale" to "de_DE" in braintree.setup() in the file spark/resources/assets/js/mixins/braintree.js. After "npm run dev" the local string was correctly compiled in app.js (inspection via Chrome developer tools). But the Dropin from Braintree where the users should enter their credit card information the language stays English.

Is there any other value to change? I found out that the english strings come directly from https://js.braintreegateway.com/v2/braintree.js, but if this is true, what is the purpose of the setting?

Thanks in advance Julian