In two of my last three stories I used similar keys,
for instance

     "  "      spoken English
     < >       spoken Chinese (Mandarin one story,
Cantonese the other)
     o O ( )        private thought
     o O > <        telepathy

with no delimiter meaning spoken Japanese.

I believe that I am obligated to differentiate language,
so it is obvious to the reader why some characters
understand certain utterances and other don't, as well
as simple verisimilitude.

I agree with that. In a fic that i am doing, the characters frequently
switch between English and Japanese, and it is integral to the plot that
some people can only understand one of the two languages, thus i need to
indicate the switch without saying it in so many words every other line.

Heh. The way I see it, the double quotes already _HAVE_ a firmly entrenched
meaning. I mean, sure the characters are speaking English rather than
Japanese, but they're still *speaking* and not thinking, 'pathing, or
writing panda-notes, right? English prose is confusing enough without
fanfic authors getting creative with their delimiters. That means that we
shouldn't change a well-entrenched format convention for a single fic,
since it will simply cause chaos.

In Philip's case, I recommend that you use a separate delimiter nested
_within_ the double quotes:

"I am speaking Japanese." (c'mon, work with me here!)
"<Now I am speaking English.>"

Double quotes do not change meaning, but you have your translation delimiters.

                           ---------------------
##   ## ##   ## #######  ##   ##  AKA Tom Jefferys, Time Lord for Hire
##   ##  #   #   ##   ## ### ###      "Have TARDIS; Will Travel."
## # ##   ###    #####   ## # ##  Wielder of ANVIL and SPAM!
### ###    #     ##  ##  ##   ##     [ Will code for food! ]
##   ##   ###   ###   ## ##   ##_of_UT haunts <wyrm@mail.utexas.edu>
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"What kind of stupid alien escapes in a car?!"
                                     -- Kizuki Mitsuha, Silbuster ch.1