For 99% of undubs on anything with a file system then yeah get source region you want voices from, overwrite things in destination region (most of the time this is English to Japanese or anything else to English). Other times voices are lost at code level which means either getting them back, or perhaps more commonly then people backport the script to whatever they want (again usually Japanese base) but that might well be a full bore ROM hacking effort.
Do the names of the files/directories/extensions not give you any idea? This can include elimination (chances are a folder called gfx is graphics sort of thing). You might also try comparing regions -- translation of games usually only does text, font and graphics with everything else staying largely the same so what differences (and they will likely be notable differences rather than a few bytes here and there) will include the language aspects.