Problem
The translation of the button «Place order» doesn’t seem to work with the current version of WooCommerce (2.4.10). It’s simply ignored.
(Possible) Solution
1. In the file «payment-method.php» look for the following code:
data-order_button_text="<?php echo esc_attr( $gateway->order_button_text ); ?>"
2. Replace the code within the quotes with the desired text. Example:
data-order_button_text="Bestellung abschicken"
Note
This procedure is a rather dirty solution. Especially because you have to redo it after every plugin update.
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