This kind of kicked off whilst I was away...
In regards to the language thing it is a WCAG 1 AAA point. Which probably means absolutely nothing to anyone here, basically according to W3C if you say you are 100% accessible you should cater for other languages. However that level of stringency goes way beyond what is generally accepted.
In regards to PAS78 and the law, I would imagine that most people here would be considered to small fry for anyone to bother, and any existing sites could be said not to count. For new sites however I would make sure that the designer contractually conforms to areas raised in PAS78 and accepts the professional liability involved.
In regards to accessible web design, of the steps that are required to make a site accessible they always make a website a better website. Comments such as "Google is a blind user", still hold water.
Last part, flash can be accessible. Macromedia did a good job of putting accessiblity features into flash, it's just up to flash developers to use them. One good example is one of the Harry Potter flash sites - 100% accessible.