Proportional chapter seats would mean 2 Board members from WM DE :) -- which leads us to the same situation like we have with en.wp at the project level.
Yeah, that's definitely an issue that needs to be addressed. They're chapter seats, not WMDE seats. There are ways of doing something close to proportional representation without giving a large chapter too much weight (just cap the votes at 40% of the total, or something, would be simplest, more complicated would be some kind of logarithmic apportionment - lots of options to discuss).
I think there ought to be an "Inter-chapter council" with representatives of all the chapters (perhaps with proportional voting, perhaps one-vote per chapter, probably depends mostly on what they're voting on - the main purpose of the council would be coordination, not making binding decisions). Perhaps they could then appoint the board members (so the membership of the chapters elect representatives who elect the board members, cf. US Electoral College). Doing it that way increases the chance of the chapters choosing someone based on qualifications, not nationality. (If it's just done as an open vote, I'm sure people will consider candidates from other countries, but there will always be a bias towards your compatriots. This bias is reduced is there is informed discussion among a small group.)