This phenomena is not limited to Germans living in the US, but can be found everywhere.
So true... I've seen it both inside and outside the US, too... sometimes I've been wondering if it might really be "too hard" for them to get beyond a certain level, or if they're really just too "lazy" to try improving their skills after reaching a basic level that at least allows them to get around more or less... Or does it even have something to do with not wanting to "give up" their "heritage"...? I've heard of some families (both here in the US as well as in Germany) where certain members of the family (mostly older generations) literally refused to speak anything but their native tongue, having their kids translate for them if needed...
Of course, taken to the "extreme" (will say, people not learning the language
at all), this is a real "hot topic" in the US these days... in our area, for example, they're offering classes and special dictionaries in Spanish for police officers in order for them to be able to "properly" do law enforcement... on the other hand, there's that "steakhouse" (wherever it is) that's all over the news these days for putting up a sign (and may be getting sued over it): "This is America: Please order in English"...
Well, even though in Germany you cannot relate one accent to one state you can relate an accent to an area. The spread of accents took place naturally over hundreds of years, but the boundaries of the German states were somehow forced on people. Thus, when you meet a person who speaks with that broad Manheim accent, this person could live just north of the border, i.e. in Hessia. However, you will be able to place that person almost correctly. What always troubles me, is when a person does not have an accent at all, which is the case in the area of Hanover, where we live.
Funny... I'm originally from the "Greater Hanover Area"; and at one point someone actually told me I had an accent typical for my home town...
Never heard of any such thing ever before or after...
There are quite a few German families who raise their kids bilingually, even though both parents are German. In most cases, if the parents are persistent over the year and don't let themselves get deterred by their children's refusal to actively speak the language with them, that works quite well.
When I was little, my mom actually started to take English classes in order to "freshen up old skills & improve her resume"... I wanted to learn, too, so for a little while we tried to "talk English" at home... well, not surprisingly, that didn't really last long...
However, even if both parents are pretty much fluent in English and really adamant about "enforcing the rules" around the house, if they're not native speakers and do have a noticeable "foreign" accent, I cannot imagine the child(ren) being accent-free... (fluent, yes, of course). On the other hand, children that are "transferred" into an environment with all native speakers (of a foreign language) at a fairly young age (few years old) will more often than not grow up to be accent-free (or have the same accent as "native speakers" in the area)... so it's not really "mandatory" to do while they're still babies. At least, as far as I know... but I'm not really an expert here - it's been a while since I was really "into" this...