These are additional messages, not switching messages to another language. All of the original messages are still in their original language - but these are spam messages.

I get the same thing, frequently recently. I wrote asking why this was, which posted alright but has since disappeared. If it is spam, Jason, what is its purpose? I imagine only a few people on here who are not Japanese, are able to understand the posts. Assuming they are posts, and not just gobbledegook. If the 'proper' posts are still here -0 where are they? I can't get anything but these supposedly Japanese missives ...

They are not meaningful posts. They are SPAM. Someone trying to ruin your experience here by making it difficult to read any messages. These messages are deleted eventually.

There are frequently also another type of SPAM message - where a user will post in English something that does not relate hardly at all to the subject, but picks up on something like "ipad" then posts something vaguely about apple support with a link to some offsite thing to increase sales or otherwise spam users here.

so... among other things, Yamaha is a technology company, no? Can't they make a spam filter so that this doesn't get into the forum?

... they have been adding spam filters here. Sometimes such that users cannot post valid posts. There's no magic wand here. The team has been working on this issue but if bad actors have enough will - just about any reasonable policy can be defeated. Sometimes this leads to unreasonable policies accomplishing the same thing - a hard-to-use message base. There are probably examples out there of open forums that are easy to use and somehow do not attract as much spam. There's not an easy cut-and-paste for those solutions.

FYI - the writing is in Chinese, not Japanese. Just FYI.