How do you say the temperature in English?

"One degree centigrade" is still commoner than "one degree Celsius," especially in the USA.
Remember that in the USA, Fahrenheit is still so normal that "one degree" will be automatically understood as "one degree Fahrenheit." Even in the UK, weather reports and forecast often give both Celsius and Fahrenheit for the benefit of the older generation who have never become used to decimal measurements.
One hardly ever hears "one Celsius degree" and I have never heard "one Celsius", although I have heard "One centigrade."
Reports in the US media of low temperatures abroad, especially of those in Canada, are difficult to interpret since one seldom knows whether the foreign news has been translated into Fahrenheit or not. Foirtunately, "minus forty" (degrees) is the same in both Celsius and Fahrenheit.

One degree celsius. Though, informally people do shorten it to 'one degree' in every day conversation.

One degree Celsius (or centigrade)

Remember that you Americans still use fahrenheit, so if you just said one degree, the Americans would assume you meant one degree fahrenheit.

Remember the Mars Observer that crashed into Mars simply because one part was built in Europe where they used metric and another part was built in the US where they used imperial measurements?

One degree celsius sounds absolutely fine. Centigrade is the same thing but I think it is used more in America when the temperature is not being measured in Fahrenheit.
Personally, I would say one degree if I was talking familiarly and knew that the people I was talking to understood the temperature measurement I was using.
You could probably say one celsius, but it doesn't sound very natural and one degree or one degree celsius make more sense. I've never heard someone say one celsius degree though.

If you're ever in an English speaking country make sure you know what temperature measurement is used there. As a general rule, it's Celsius for most places and Fahrenheit in America. (But Fahrenheit is still used in the UK alongside Celsius.)
Hope I helped!

You would say one degree celsius. However, be aware that not all English-speaking countries use celsius. America uses fahrenheit.

1 degree celsius though the majority would say just, 1 degree.