Hello,
everything is just in 1 file and it’s just few lines of code, so if you’re a programmer you can put some breakpoints and logs there and test it.
There is no debug mode or any log by default.
But this whole thing is very tricky… I completely rewrote this plugin few times from the version 1.0 to make it works with different translation plugins, permalink structures and other things…
Yes there are probably still some edge-cases where it will not work, especially when there is another plugin altering rewrites or permalinks.