I have heard at so many occasions, especially in Germany, that people - frequently including secondary school teachers and students - pronounce words containing the letter v as if the letter were a w, e.g. willage (instead village), adwantage (instead of advantage), etc. Does anybody have an explanation for this?
It's a form of hypercorrection.
In German, there is no /w/ sound, and the /v/ sound is produced from the letter w. Hence the translation, and cognate, of welcome is for example willkommen pronounced /vɪlˈkɔmən/.
This at first leads German speakers learning English to use /v/ (or perhaps /ʋ/) when they should use /w/.
Then they decide "ve vill have to do better", and strive to correct their /v/ to a /w/, but in so doing, they over-correct and end up using /w/ where /v/ would have been correct. They can even end up doing both "ve vill go to the willage".
Some other Germanic languages similarly either have /v/ for w (e.g. Dutch, welkom) or else have a v pronounced /v/ where the English cognate would have w pronounced /w/ (e.g. Danish velkommen).
Of course, English speakers learning one of those languages are liable to the equivalent mistakes on their part.
Peter Shor's comment above points to an additional pressure: According to Wikipedia, "[ʋ] is occasionally considered to be an allophone of /v/, especially in Southern varieties of German". If true, then this would definitely be another influence, since [ʋ] isn't found in most dialects of English and sounds like /w/ to English ears (it does pop up as a pronunciation of r in some dialects, so e.g. Jonathan Ross—who has such a dialect—is mockingly called Jonathan Woss because to most other English speakers that's how his name sounds when he says it). I am only going by Wikipedia on the point of there being German varieties that have such a [v]/[ʋ] allophone, but it would certainly also lead to "willage", etc.