I visited México City fall of last year and had a great time.
Language: Use as much Spanish as you're able to. Chances are, the people you come in contact with will understand English, but not French or Portuguese.
Altitude sickness: I did not experience it during my time in the city.
Safety: I stayed in the Roma Norte area. It felt safe enough to walk around the neighborhood, especially considering there were a lot of people out and about. I took an Uber everywhere else.
Condom use: I always bring condoms whenever I travel. Did not use them with any of the guys I was with though, both for paid and unpaid sex. Lube was used 100 percent of the time.
Where to stay: Roma Norte. I like to go out and enjoy the nightlife, but I also want to be able to sleep without having to wear earplugs. Go with whichever accommodation provider you like. I stayed at a Mister B&B for the first half of my time there (host lived in the property) and hotel the remaining half.
Water: Assume the tap water will give you the runs; don't brush your teeth with it. Fortunately, my MBnB host has filtered water and the hotel I stayed at has a dispenser. I bought a few bottles of sparkling water as well, just because prefer it over still.
Food recs: Churros from Churrería el Moro, tacos from Tacos La Chula Juarez and Taquería Orinoco, simple food from La Herradura.