Touch Kobo sideloaded books and Japanese dictionaries

So after a lot of hassle, solved by friendly MobileRead denizens, I finally got my Kobo to play nicely with sideloaded Japanese books from Rakuten, and downloaded and installed the Japanese dictionary. And then discovered the dictionaries won’t trigger.

With English, including sideloaded books, long press triggers the dictionary for the word I’m touching. With Japanese sideloaded books, it doesn’t – I can’t select text at all, for any function. I don’t have any sideloaded books containing both languages so I can’t test that.

I have seen mixed reports about this; some sources (http://blog.the-ebook-reader.com/201...eatures-video/) mention the issue being fixed so sideloaded books can now be used with the dictionary. It's a niche problem so there's relatively little discussion, which makes it harder to be sure.

I also don’t know whether this links in with complications around Japanese, as it’s reported widely that the Kobo Japanese dictionaries are weird in various ways, and I don’t know to what extent this may have changed, though there is at least now a J-E dict available and I have this.

I’ve tested changing the Kobo language to English, but this makes no difference. I’m not sure if there’s a way to designate a specific book’s language within Kobo – I’ve searched around but not seen anything. Mine were sideloaded via Calibre and are set as Japanese there.

Can anyone advise on this? I don’t know what kind of problem it is, so it’s hard to know what to do. This is a fairly big drawback, and that plus regular freezing is prompting me to consider dropping the Kobo entirely.