I created a very simple Workflow that has the following task: If the field "Worldwide?" equals Yes ("Ja" in German) it should send a Mail. But it doesn't work and I have no idea why. I tried the Mail-part individually and that worked. Do you have a clue why it doesn't work?
Make sure there is no space or hidden character in your if item equal to yes.
As your saying the email sending is working without if condition, then try to send simple email without any condition and in that email body add field reference item (yes, the column which you are trying in the if condition).
Now, you can see how exactly the value is coming for that item, then accordingly add that again in the main if condition.
It should be caused by the If condition.
Please use "Log to History List" action to log the value of the field "Worldwide?", check if it has a different format with the value that you set in the If condition. Then set the value in If condition using the logged value format.
You can also add a "Log to History List" action before the "Send Email" action to track if the condition is triggered.