The OCR Settings dialog box serves for changing the recognition language and the reject character. You may launch this either from Power PDF (in the Convert Pages dialog box) or Convert Assistant.
To achieve the best accuracy, select only the languages your document contains. Multiple language selection is allowed, see Multiple language selection for details.
You can set Automatic Language Detection, then the program itself will decide the language of the document, choosing from all languages with a dictionary. See Language Selection for detail on how it operates, along with a full list of supported and dictionary-supported languages.
Unrecognizable characters are represented by a reject character which is a tilde ~ by default. You can change this reject character if you wish.