Allow import and use of EPUB documents directly.
If you allow import and use of EPUB documents, i.e. give Readlang the function to be an e-book reader for EPUB format (as well as text format). This would save having to convert to text in Calibre (not an onerous task admittedly).
I think what lies behind my thinking is more that the user will benefit from the look and feel of the e-book he may be used to already, and also have benefit of any chapter-headings or illustrations.
I notice that for example Calibre does not have support for foreign language dictionaries (it does allow English language lookup in a PD dictionary), so that would make Readlang quite a good competitor in the e-reader software market (even if it doesn't have all the other tools that Calibre has).
I don't know how difficult this would be. EPUB is an open format (although I don't know how well the e-book producers stick to the standard. There is also DRM to worry about (maybe)).
There will also be a space penalty. - oops, maybe not. Just looked more carefully at the folder where I imported the book I'm currently reading in Readlang, and the .txt version is actually bigger than the EPUB, which is (to me) counterintuitive. Interesting. So there may not be a space penalty, but a slight gain.
The reason ePubs can be smaller than txt files is that they are compressed. If you rename an ePub file to .zip you can look inside :)
If I support ePub it will certainly be DRM free books only to start with. Savy technical users can easily remove DRM for their personal books using Calibre.
To support DRM ePub books seems to be very expensive. I looked briefly and it costs $10,000 upfront for Adobe Content Server plus a charge for each book distributed. So I’d need to be very confident that Readlang could make money before going down this road.
Don’t want to promise a timeframe, but I’m going to start working on DRM free ePub support soon.