English is likely the fallback for the string values throughout the code, so it's possible that an English file just isn't needed.
Have a look at the README here:
https://github.com/residualvm/residualv ... residualvmRun step 1) and generate a new template file.
Try to compare the newly generated residualvm.pot file against the German one. Maybe some lines are new, and are missing from the German translation?
There is always the possibility that something didn't get localized, but rather hard-coded.
What is the text for the non-translated string you found?