Strange can be a strange word. If I asked you to explain it, you’d probably have no problem. It means weird, unusual, not normal. Easy.
Now though, think of words related to strange.
You can probably think of strangely, stranger, and estranged. OK, strangely is the adverb form of strange, but what’s the link with the other two words?
When we go back to the words’ Latin origins, things become a little clearer. All are derived from the Latin extraneus, meaning external. You can probably think of a lot of modern words beginning with ext- that share this sense of being outside or distant: extremities, extract, extradite etc. Similar words derived from extraneus also took a slightly different meaning, of being above or beyond the norm: extra(ordinary), extreme, and so on.
It’s not too hard now to see how strange could share an etymology with such words. If something’s strange, it’s outside the normal order of things. Perhaps over time early forms of strange dropped the “eck” sound from the beginning as the meaning of strange became quite distinct from the basic concept of distance or being outside.
Of course stranger and estranged still demonstrate the more basic sense of externality. A stranger is someone outside of your circle of friends and acquaintances, and if a couple become estranged, they separate from each other, and become like strangers.
Understandably, these words can be quite confusing for English learners. I’ve often had to tease out the distinction between a strange person and a stranger, for example. This confusion is compounded for speakers of the Romance languages, all of which have words derived from extraneus to mean foreigner (étranger, straniero, estrangeiro, and extranjero). It’s particularly confusing for French speakers, as étranger means both foreigner and stranger.
So of course an English teacher also has to often explain the difference between foreigner and stranger. Like make and do, it’s another of those curiosities that we might never expect could be an issue for learners, as the words are so distinct in English.
That’s why it’s not necessary to have learned another language to teach English, but it helps. Specific knowledge of your students’ mother tongues can obviously help, but simply the experience of being a stranger to a language helps to put you in a student’s shoes, and make you aware of just how strange English can seem to them.