Here's the developmental side of language...
our mother tongue is learned by immersion from pre-birth (in utero hearing begins around 6-7 months gestation) to around age 6 years...
From birth to 12 months a newborn baby makes all the possible sounds of every language on earth (think those 'clicking' sounds that are in some African languages, or the intonations of Mandarin, or the alphabet sounds of German or English)...this is the 'goo gah' of babies...
From around 6 months they begin to repeat more often the sounds from what they hear around them.. they 'pare back' those sounds...then the first 'word' appears around 12 months. It's usually a really functional word (Mum, Dad, Hello)...
Deaf babies, and babies with speech problems usually make sound to 12 months before people start to notice that they are not communicating (the exception would be a child who has something wrong with their speech apparatus)...
You learn your language by the sounds of those around you... but by age 6-8 the capacity to learn a 'mother tongue' begins to switch off - you can no longer make all the sounds even if you wanted to - in fact, it can be like the person can't actually 'hear' the sounds of a new language - example is Mandarin and Japanese where they do not have a sound for our English - for example, my Japanese friends call me 'Jesh-Ka' because they don't have a sound for our hard 'ss' or 'si'. The mandarin-speaking children I teach have trouble with the sound 'b' and 'd' and they will substitute the softer sound 'p'.
I think the problem you are describing with 'V' and 'W' is that firstly, there isn't actually a 'W' sound in German...and my husband just pointed out also the hard 'D' sound isn't there either - our son is 'Edmund' but he becomes 'EtMoont'
So, even if the person 'knows' that the sound is different, they may not actually be able to make that sound with their mouth! - It's the same for me if I try to speak Mandarin - there are at least 7 different intonations for the same sound, and it changes the meaning - without the intonation, you can, for example call someone a 'mouse' (Lau-Shu) or a 'teacher' (La-Tsu) - well, someone can correct me on how to reproduce the sounds and the spelling of the sounds...but that is what is 'sounds like' to me.
Hope that helps.
Oh, yes, and we notice here people speaking German with a 'scottish' or an 'english' or an 'Irish' accent - just as you get people who speak English with a 'chinese' accent!
Partly it is that we apply our intonation and sentence emphasis to the new language, part of it is that we are dropping in sounds from English to words in our non-mother tongues.
Oh, and mother-tongue is defined essentially as the language/s you learn in the first 6 years of life - after that age, you'd be hard-pressed to find someone who speaks the language technically perfectly...but it's possible to have several mother-tongues - those languages you heard and learn as a baby/child...
Hope that all makes sense/helps...