As we have many relations living in Spain including grandparents and cousins and with myself and older dd speaking Spanish (although mine admittedly rusty these days), we obviously want Spanish.
mariposa, I don't know if this is of any help, but I know for a fact that this would be considered a valid argument at DCGS – while trudie is correct to say they are allocated a language as a form group, there are at least 2 boys I know of who successfully asked that their DS be allowed to take Spanish for similar reasons to yours. I don't think it's bizarre to allocate randomly in the first instance, but I do think it's bizarre not to listen to specific and valid arguments in a particular case.
The HT's argument is very odd – if all the girls want to study Spanish, you hire more Spanish teachers and fewer German teachers! This is perhaps why not many DCHS parents I know are all that sorry that he is leaving…
I would take Guest55's advice and pursue this one if I were you.
(ccl, a lot of the Upper Schools just don't have sufficient MFL teachers to cater for DCs to take more than one, and the demand, generally speaking, is not there to warrant hiring more. Incidentally, DCGS does not teach Latin, although they will facilitate it as a twilight option)