Language Files Override?

I have added the official German language pack by @osworx and now I'm wondering if there is a way to override language definitions (for the English default language pack, as well as additionally added lanuage packs) in case I want to change certain terms or phrases, so that my changes do not get overwritten when performing an update.

You could use vQmod to add entries to the bottom of the language file.
So if you want to change "Add to Cart" to "Add to Basket" on the product page, you could use vQmod to insert a line like
$_['button_add_to_cart'] = "Add to Basket";
You would just add new lines as needed.

+1 for Qphoria's method. It's way easier than editing the language files directly, and the updates don't get lost when you upgrade OpenCart.