1 - don't run it on the ISO, run it on the extracted file. (As well spotted & re-stated by KingMike.) Have you done that?
2 - get familiar with Japanese, enough to recognize "garbage" from potentially meaningful Japanese. (Ex: kanji are often intermixed with hiragana/katakana. And punctuation is common in sentences - even if it is different from English punctuation.) Then take the time to browse the entire dumped files and look for "the real thing". (Google translate can help verify stuff makes sense, sometimes but not always.) You could try and look for actually existing game messages you've seen on screen, if you take good care not to mistake one kanji for another, and it you choose a sequence that doesn't involve English characters (just in case - we don't know how they are encoded and you chose to not dump ASCII).
Just for comparison, if relative search doesn't work, then you should be browsing the entire extracted files.
EDIT - I forgot to mention that you have to set the encoding to SJIS when you browse the dumped files.(Ex: in Notepad++, just after opening the file and without typing anything in it, menu->encoding->character sets->Japanese->SJIS)