Text inside the \colchunk command (which is used inside a parcolumns environment) escapes the diffing algorithm. Text that is inside a parcolumns environment but not in a colchunk is treated correctly. But \colchunk is essential to the core functionality of parcolumns. A minimal working example is attached.
As it is usually better to fail to mark-up a part than produce an unprocessable output, latexdiff treats all unknown commands as 'unsafe' and with atomic arguments, i.e. it should neither be marked-up nor should it's arguments marked up.
With the option append-textcmd it is possible to tell latexdiff to treat the arguments of selected commands (+ a lot of presets) as text, which should be diff'ed. So --append-textcmd=colchunk will probably work, if colchunk is used directly.
But latexdiff is not a latex process so it will nor realise that the same applies to custom command \bilingual.
You can use --append-textcnd=bilingual but this will only treat the second argument (here the English text) as text, not the first. This limitation is difficult to work around, as it would essentially require a database of commands and which of their several arguments are text and which arent, so I am not planning to address it in the foreseeable future.
As a work-around you could add spaces between the \bilingual command and its arguments, which fools latexdiff into processing the arguments as normal text; this method is not entirely safe, though, as some changes will lead to a diff file that cannot be processed.
If the append-textcmd approach works for colchunk, I could hard-code that behaviour into latexdiff.
