It is good to know that Ubuntu MATE is installed for second-hand computer sales. I'd be very happy to stop by when I am in Japan next time. I wish there was an international directory on the Ubuntu MATE website with Linux-friendly hardware stores. Is there any chance to get your store listed, perhaps as part of an Ubuntu MATE Market?
Some of your customer's problems may indeed be related to the lack of Japanese translations of the Welcome application. Nevertheless it is important that the Welcome application provides more general information on how software can be installed, what different sources and file names there are, what kind of software centers are available and what the advantages and disadvantages are of using them. This information could perhaps be added to the introduction and not just to the Software Boutique. I also prefer the Advanced Menu to be enabled by default.