Yes, January is a great time to do a South America cruise. (We have done it) And yes, winter in the northern Hemisphere is indeed summer there. You will probably be cruising form Valparaiso Chile to Buenos Aires Arg. --or visa versa. It will be pretty warm in both places. but not really hot.
Punta Arenas, Ushuaia and Cape Horn However are very far south. Cape Horn is about 57 degree south, roughly the same as Kodiak Alaska in July in the North. So it can be pretty chilly even in the summer. but not dead of winter cold. The weather around the Horn can be nice and calm, or ghastly, or anything in between. Our cruise went on south to Antarctica and the waters between Cape Horn and the Antarctic Peninsula are always among the world's roughest.
I would recommend Holland America or Princess for this cruise. Don't know if Celebrity does them. I know nothing of Azamara.
The shore excursions range in intensity, some are very easy some are tougher. They will be identified as to which is which.
Lack of Spanish will pose no problem for you at all. It is a great cruise. the scenery on the inside passage between Puerto Mondt and Cape horn is unbelievable