Th and Thz

If you are not trying to eradicate your 'f' sounding 'th' then you will end up sounding like a child, since English speaking children exhibit this problem universally up to about the age of three or four. Also, you might be grossly misunderstood, even evoking a punch in the face!

Consider this common polite phrase, "Thank you!" If your 'th' is still an 'f' then it sounds like "Fank you!" Add the other common problem that vowels in Europe are darker sounding and the 'a' sounds like an 'uh'. Now we have the English speaker hearing "Fuhnk you!" Since the 'n' in European languages tends to have more of a nasal quality than in English, the English speaker may not hear it so well. Finally, just before he hits you in your face, he hears you say, "Fuhk you!" or more properly spelled, "Fuck you!"

You see why pronunciation can be critical to your health and good relations with English speakers.