German pronunciation is very specific and there are enough rules to it that once you've learned them, you can pronounce the majority of German words, whether you know the meaning or not. English, on the other hand, has a mind of its own and it can be difficult for those learning English as a Foreign Language (EFL). Hey, some words are tricky even for native speakers.
And there is one thing that's very important to Germans: not showing their tongues.
In five years living in and traveling through much of Germany I never once saw a kid stick out their tongue, nor an adult do the same thing in jest. They don't show their tongues. It's extremely rude to stick out your tongue at someone in Germany.
So when Germans speak English,
they don't want to be rude, so they carry this respect over into English and suddenly "thing" becomes "sing" and "Thursday" becomes "Sursday." It's the equivalent of an English speaker trying to pronounce "pf" as in "Pfanne." It's hard!
The politeness which lead to Sursday instead of Thursday is so ingrained in the German culture that they don't even realize that is what is holding them back. However, when they get "permission" to show the tip of their tongue, to really feel the difference in the musculature, they get used to the feeling and they suddenly acquire the skill of pronouncing "th."
And we're back to Thursday!