You can use Squirrly SEO plugin in all countries and all WordPress languages.
So, yes, Squirrly SEO supports Japanese. From keyword research, to focus pages, to live assistant and to technical SEO.
You can use the Keyword Research tool from Squirrly SEO to search for keywords in 140 countries and Japan is in that list. Our Bulk SEO feature works with all WordPress popular multilingual plugins and you can optimize the SEO Snippet in any language.