I'm also joining this thread late, but it's interesting.
If anyone knows good books about bilingualism NOT about OPOL, but rather two parents of same mother tongue living in a foreign country, I'd love to know.
However I can only read in German or English, my French is far too rusty.
Both dh and I speak German (mother tongue) but live in the UK, and the children attend an english school.
Sometimes I think they have two weak languages.
Is it true that the strong language is defined by which language you do maths in and calculate?
Is there a definition at all?
I feel the English influence is extremely strong and they pick up English a lot quicker, however there are things they'll never know as well as a mono-lingual English child.
Their German is certainly the language they understand better, but not necessarily speak better. If they make a mistake in German, it "sticks" for years and they will use the wrong form over and over again, no matter what I do. Ds1 can now even read in German, but this doesn't seem to make a difference.
Both dc 1 and 2 struggle with writing even in English anyway so I haven't made them learn to write in German so far, although ds2 is quite interested and we've started doing some German workbooks. Time is very limited after school and after school clubs, so it's difficult.
What strikes me is that I know some adults who have grown up in the UK with German parents, and some of them speak no German at all and can only understand bits and pieces, or they can speak German but with an extremely strong accent and lots of mistakes. Among them people with German mothers (the person who stayed at home at that time when they were young) or even two German parents.
It's worrying so I'm desperate to get it right, however I also don't want the children to be disadvantaged at their (English) school, so obviously I don't want to ban English books etc.