I know this has been bandied around but having just gone through most of the file again, the English really needs fixing, at least the strings which are given out to the localizers.

I'm a tad tuned out, I believe we had just the Default language but now we have Default and English? Or is that the British English file on Transifex? And if so, should we not be using those? There's some really crazy strings offered up for translation like tsBoardConfig.Caption which is "tsBoardConfig" or tsWordCheckMode.Caption which is given as "tsWordCheckMode"

I translated English as American, and fully agree about proofreading. As I wrote on Transifex it would be a good idea to double check own localizations with this translation, either.
Some strings are less important than other. ts*.Caption are tabsheets which are not shown anymore in the options dialog. tb*.Hint are important because it's the (unlabeled) toolbar button. I added a few remarks and could do more, if you think so.

My thinking is to take the English file and make it match the German translation, which is very good (I think Bussinchen did it). I could do that if you tell me what the best way of doing that is. I mean, which file should I proofread?

I think it's important, Transifex is a much more public place and we will probably attract other translators who know little about the project, so they need good source strings.

IMHO we should keep the original strings since any change is applied to all translations (except of big mistakes). You are welcome to proof read and verify "English (United Kingdom)" translation (I guess it cannot be changed to en_US). Perhaps remarks or annotations (Anweisungen: in my German interface) might help new translators.