I don't often run the Diagnostics on its own, I normally let Pack & Go take care of it, but when I ran it this morning I noticed the 'optimising' option was listed twice, in British English and again in 'American' (see image). The 'British' version was ticked, as if it had completed, but the other was still doing its thing.
I assume it's not actually running the optimisation/optimization twice is it?
The en-uk translation is unmaintained. I have removed it for the next release.
If you delete the file (while IMatch is not running)
c:\ProgramData\photools.com\IMatch6\resources\imatchres-en-uk.json
the problem should go away.