I would change that up and require that a mid level have some proficiency with a second language. Preferably one with some notable differences as compared to the one they know. Like static vs dynamic or functional vs imperative.
If there is anything even close to linearity between the "levels" of software development proficiency, then "Will understand code found on the Web before even thinking about copying & pasting code." is not equidistant from senior engineer as it is to "Might copy & paste code from the web (such as Stack Overflow) without understand it, if it just works"
Being a Junior developer should be a proud accomplishment, not a title for what you can buy for $10k and sitting in a classroom for a few weeks.