Only a guess but as Ti-ejtsch says above, different brands and only burning at lowest speed does make a difference. If you deleted the directories without a problem, saved the new ISO image without the additional languages and burnt it to a DVD all without error, then I would think its possibly just a bad burn & try again.

Seems most posters here seem to like DVD+R but I had a couple of problems once and tried DVD-R which I thought seemed better but I still think it probably doesn't make much difference if they are good quality DVDs.