To be a polyglot is alluring

To be a polyglot is alluring to some people, including myself.
I used to believe that taking the linguistic knowledge as the drug would ease the pain during the learning (or better: acquisition) process.

Do theories of SLA (second language acquisition) help? Are linguistic typological knowledge useful for leaning a new language? How about phonological, syntactical, and semantic knolwdge? How can they be "packaged" systematically, so that we can go further without much pain along the way of becoming a polyglot? In addition to linguistic perspectives on SLA, would the tools and resources proposed and developed by computational linguists do good to SLA in practice as well? How?

Muddled suddently, lots of works to do.