Problem is that I'm not sure I'd be able to convey that message to someone even in my native language (it's a Romance one). I'd probably use two archaisms for "smaller pieces of wood" (probably the "tinder" and "flint" words you mention) and then I'd use "small rock" and "rub the thing between your hands until you get tired of it and then use a lighter".

There's lots of words in my native language of which I know the meaning of, technically, but practically speaking they might as well be Greek to me.

There are all of these types of flowers, trees, countryside-related mechanical terms (my peasant brother knows the names of all the pieces of which his wooden-made carriage is made of, I have no such knowledge) which might as well be from a foreign language to me. They sound right but I don't know exactly what they represent.

I'd say that you can call yourself fluent in a language once you can express feelings like anger, love, nostalgia etc. and can understand and make jokes in said language.