I'm writing a résumé for the first time and I want to convey that I have a near-mastery of a programming language. Most sample resumes use "proficient", but that sounds like I'm not very good with the programming language. I have almost 10 years of extensive experience with the language, so I feel qualified to claim that I have "mastery" of the language. However, I don't want to use "mastery" because that sounds arrogant.
What's a word with a meaning between "proficient" and "mastery"?