Games are locked by television regions. NTSC-U/C applies to USA & Canada, NTSC-J to Japan and PAL applies to a set of countries, which include the United Kingdom and Italy.
The language depends on the publisher/game; I have seen games have different amount of languages...
The Wii VC games technically shouldn't obey the Wii sound settings because each game is suppose to be a direct port which is ran on top of an emulator. To my knowledge, the hardware for most (if not all) of the consoles that are emulated on the Wii did not have a stereo/mono option.
Instead, the Mono/Stereo output was controlled and toggled from within each game. As an example, I seem to recall the Final Fantasy games for the SNES had these settings in their options from within the game play menu.
I never played Super Mario World much as a kid (didn't own a SNES :( ) but Nintendo has always tried to spot-light the quality of their new hardware by the release of a few games that integrated most of the new technologies provided by their new hardware.
I think the SNES was the first Nintendo console to use stereo sound. Since, SMW had stereo sound, I doubt there was a way to cut if off because Nintendo would have wanted to show off the systems audio capabilities.
SOLUTION
Sorry for taking so long to respond to this with, um, an actual solution but there is a hardware way to fix this.
Plug your left and right audio RCA cables (which I assume you are using) into an RCA splitter. You can then plug your one cable into your working TV channel and you will get both audio channels to play through the one speaker.
Here is an example of what you are looking for. As you can see, these things are dirt cheap, and if shipping isn't an option, you can probably find one in a local Walmart, Target, RadioShack (or international equivalent.)
I actually have a couple of these splitters and they were necessary in my childhood because I only had a mono-input TV in my gaming room. I actually daisy-chained some of them because I had a bunch of gaming systems as a kid and I didn't have a multi-input box. To much daisy-chaining will degrade the quality of the sound, but I'm guessing with your setup that's not going to be an issue.
Best Answer
You can't wipe your Wii without wiping the VC games, and they can't download them without having full access to your Wii Shop account and any credit card information it stores.
You can't sell a Wii with VC installed without handing the buyer considerable amounts of personal and financial information.
It's also, aside from that, against the licensing of VC content to resell it: it's explicitly non-transferable when you buy it. As a consequence, it will come as no surprise that Nintendo has set up the system such that you can't clear personal info off a Wii in preparation for reselling it without also wiping the Wii Shop downloads.