It is said on this website that it is possible to change the language of Fallout New Vegas Ultimate Edition by modding the ISO file:
Games Xbox 360 Info
SS VERSION: Version 2
WAVE VERSION: 13th Wave (XGD2)
RELEASE NAME: Fallout New Vegas Ultimate Edition XBOX360-SPARE
REGION: PAL & NTSC-U Only
LANGUAGES: English . You can enable other languages but you'll have to change a couple of files in the ISO to do so.
I got the same release and it is not in english but spanish despite it labelled [MULTI]!
My Xbox 360 dashboard settings and locale are already set to English and USA.
I would like to know how to enable other languages either by modifying the ISO or the extracted files for RGH format (.default.exe).