The funny thing is that the natural answer to "How do you write that?" isn't usually a string of letters. You can separate the syllables if you think that will help, and usually point out stuff that could be potentially confusing (B vs. V, for example). The source language is completely irrelevant, if I'm asked about an English word I'll "read it in Spanish" and that's usually enough. A friend and I tried to teach our bilingual English-Spanish friend to do this, but he just couldn't do it; he'd "spell" it in English, and we'd get lost in the third letter at best. Then I'd say it as if it were a Spanish word, and my friend would go "oh!" (She was the one who knew less English).
There are exceptions, of course. I can "spell" my name on my sleep, because it's weird and tends to confuse people, so I'm used to it. My friend with a Swiss German last name automatically spells it as well; but for her name she'll say "Sara con hache" [Sara with an H], and not S-A-R-A-H. (She's occasionally been asked where to put the H though)
So, I was wondering... are there more languages whose speakers do this?
Note: inspired partly by some posts about whether Spanish is phonetical or not (I don't think this shows it is, but that we do consider it that way). Also, callunav's posts made me think about it again. I suspected her conversation had been in English just because of this.