I think it's possible to gain substantive exposure to one new programming language a year, to kick the tires and gain insight into some of the features and idio

I think it's possible to gain substantive exposure to one new programming language a year, to kick the tires and gain insight into some of the features and idioms that make it tick. However, I don't think it's really possible to become an expert in one new language per year. I think it is much more realistic to master one new language every 3–7 years (depending on the language). Playing with one new language per year might help one figure out which new language to master.