I've been going with you buy your native language, as well as foreign ones. IIRC, the skills run 1-12 in scale, so a basic idea of proficiency might be as follows:
Skill level 0 (DLPT score 0 to 1): huh?
Skill level 1 (DLPT score 1+ to 2): very basic unde4rstanding; a few words
Skill level 2-3 (DLPT score 2+ to 3): suvival level. You can get by but lack a lot of nuances.
Skill level 4-6 (DLPT score 3 to 3+): proficient. You can handle most situations and dialogue. Problems with obscure or technical words. Mos tnative speakers fall about here.
Skill level 7--9 (DLPT score 4 to 4+): expert. No problem with grammar or vocabulary. Capable of multiple dialects with ease. Well educated native speakers fall about here.
Skill level 10-12 (DLPT score 5): complete command. Near impossible, but it could happen; even educated native speakers are rarely this proficient with their own language.
*DLPT refers to the Defense Language Proficiency Test of the US military. Most non-native speakers achieve a 2 or 2+ and can do well in-country; 3-4 can often pass for natives. 5 seems to be there more to round out the scale; I know no one, even a native English speaker who can mimic perfectly multiple dialects, who scored a 5.
Figure difficulties as TN5: ordinary conversation "Where's the bathroom?", TN10: politics or other abstract but common things; TN15: scientific or technical conversation in your field; TN20 scientific or technical conversation outside your field; TN25: if you're trying conversation this esoteric you probably have no friends to talk to... 
Hope that helps.
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill