Thanks for the replies! Languages are a very tricky and ESID matter, I've found, and I appreciate hearing from everyone.
My personal language experience is uneven, to say the least. I've taken 2 semesters of Italian, 1 semester of French, 2 semesters of basic Latin, 1 semester of Medieval Latin, and 2 semesters of graduate-level Old English, as well as several courses with Middle English literature. The lack of modern research languages is what really motivated me to ask after others' experiences. Oh: and that I took Italian and Latin P/NP. I have a personal philosophy that the grade is irrelevant, and it's the command of the language that ultimately matters, but it does leave me in the lurch for "proving" anything at application time.
I've heard that trick of using untranslated sources in the writing sample, and it sounds solid to me (especially given the lack of grades I have for the language courses). I've also heard about taking the SAT II in Latin. How do schools typically feel about independent scholars seeking to take their department's proficiency exams? I mean, I'd be happy to make an appointment with a language department chair to take a graduate-level translation exam, but would it even help? Are they likely to even talk to me about it?