I'm interested to run these games, but I only have Japanese copies of the games.

As far as I know, Sly 3 wasn't initially released in Japan, so until recently we were under the impression that there was no japanese dub. However, it was brought to my attention a while back that it was dubbed for the release on the HD remaster in 2010/11 - but I guess I forgot to add it to the list of variables. I'll add it now.

Yeah, Sly 3 never got a PS2 release here. Now with the HD collection they dubbed the other 2 games complitely again and Sly 3 with it.

Could perhaps Japanese be faster than Dutch...

Don't think so, there isn't digital versions of Sly games in Japanese.