I agree with the spirit of your point, but I'd be hesitant to say you're "mastering" your 15th language if they are all jumbled together. It sounds like you've learned 15 languages, but mastery implies an extreme depth of understanding beyond the point of confusing minor details. I would bet you have at least one or two languages that you work in often that you never get this stuff confused for, and those are the ones you've mastered. The additional languages are subject to a sort of Jack-of-all-trades syndrome.