1)Is this the way to talk about the temperature : "It is 20 degrees Celsius." ... but perhaps excessively correct. Most speakers of Br Eng would say "...twenty degrees Centigrade" (much to the annoyance of many purists!). In the UK, younger people (born, I would guess, about 1970) would omit the last word (whichever) entirely, never having known any other scale.
2) How do you pronounce "St. John." Like Saint John ?
... although sometimes, in surnames, it is /'sɪnʤǝn/ (with stress on the first syllable).