"So for example, how would a non-Japanese pilot get work in Japan? I suspect JCAB tests are conducted in Japanese?"
We undertake an abbreviated JCAB ground school (and examinations) conducted in English.
"For instance, could I not obtain my ATPL here in UK and then apply for a direct entry pilot position with JAL or ANA??"
Not for direct entry - you`d have to apply through an agency, and have at least the minimum experience levels on type, and minimum total time. These are contract positions vs career conditions - ie. as acontact pilot you will not be entitled to any career advancement outside that for which you have been contracted.
"As for the recognition of PPL only, Japan is a member of ICAO so there is a recognition protocol between members and there will be a conversion procedure within the Japanese regulations."
In that case, the F/O`s employed here are going to be severely disappointed to learn that they have been given the WRONG info by the JCAB, and needlessly wasted thousands of dollars!