I am from Brazil, and change the voice Garmin Mobile XT 5.00.30-wp-asn to Brazilian Portuguese... (I think others languages can to do too...)
Only copy the .vpm brazilian voices file to directory \Garmin\Voice\ and change its name to English_American_.vpm
Of course, I do a backup of English language, but it runs cool. My GMXt speaks portuguese now.
Garmin Mobile XT 5.00.30 wp_asn runnng from SDMMC\Garmin
Que.exe modified with UltraEdit, as oriented here in forum
garmin.mscr adjusted too (portsplitter and the corrct name "queCE5.exe")
It was a hard battle....