Needs to be intelligible and without misinterpretation. An f instead of a Th is unlikely to be a medical complaint, more a regional accent. I would expect correct pronunciation of numbers and words as per published RT guidance for the purposes of examinations. We are advised/instructed to use unusual pronunciations for clarity (five and nine spring immediately to mind) so insisting on correct procedure is fine. True speech impediments are a more knotty subject but anything that precludes making yourself appropriately understood on the R/T might count as a medical bar to solo flight