You can easily have a text in English with quoted sentences in French or in Turkish, using the same font. Try the same with e.g. Russian and Greek and see if you will be able to read the result. Turkish and French alphabets are Latin with some diacrytics added; current Cyrillic is much more distant from Greek than that, as you bloody well know.
 lowercase glyphs aside, (И, Н) and (Η, Ν) alone are enough to render the result unreadable (shift circa 16th century, IIRC; at some point both Eta and Nu conterparts got the slant of the middle strokes changed in the same way, turning 'Ν' into 'Н' and 'Η' into 'И')