Sombra hasn’t had much time alive. In the time she was created/brought to life by Junkenstein and the time she meets and goes with the Undead Gunslinger, it’s about a month in total. In that time, she absorbs everything like a sponge but she’s still relatively new to everything. In the time that she stays with McCree and gets to know him better, it’s about another month; no where near enough time to talk fluently and understand everything but she’s more responsive than she was a month ago, much to both their relief (McCree would become irritated and Sombra confused which would lead to bouts screaming).
When he starts amping up the “charm” output from 1% to a solid 2…she reacts accordingly now. He decides to bring her flowers, as he has in the past. Before, when he would do so, she’d crush them in her hands. Now, she takes them and brings them closer to her nose, despite not having any sense of smell, breathes, and smiles, no matter how crooked and awkward and says “Glad…” She talks in concepts more than words but he’ll take it over grunts and hisses.
It’s not anything like the Undead was imagining when he sought himself a companion but when most, if not all living people think you’re an abomination, seeing someone smile at him has him feeling at least a little bit less angry than he usually is.