Surprisingly I agree with Andreytcho.  My German didn't solidify until I lived with a family. The kids had a big impact because they didn't speak English.  My language skills improved dramatically when the highly articulate grandmother would speak nothing else with me because she felt my German was not developing quickly enough.  Andrew's Russian didn't solidify even after three years in Moscow because he wasn't really using it enough.  And yes,  jet-lag can effect language.  I remember returning directly to school from Germany in a jet-lag haze and reading The Last of the Mohicans for American Lit.  I kept substituting German words for those coming out of the mouth of native-American Chingachgook.