Looks like this is only problematic with curly quotes. With straight quotes the colour for non-translatables is displayed:
degrees Celcius is in my list of non-translatables
it's listed in the F4 pick list
however, it's not colour-coded in the Source segment pane:
Ie bien a fleissieka sammla
Hans CafeTran Wiki
If I'm not mistaken, CafeTran now respects word boundaries in the Source segment pane – which is great.
However, when a non-translatable is surrounded by quotes, it isn't colour-marked: