I learned metric in school and used it in college science courses, but didn't become fluent until I lived in Germany for a few years. It was trippy having "non-science" people talking about grams and centimeters. Metric fluency has come far more easily than German.

Oddly enough though, I've found that I react like the Germans around me to weather forecasts in Celsius (30 degrees - it's going to be HOT!) when if I heard it in Fahrenheit, I'd react more like a Texan (86 - pleasantly mild for a summer day)