Even if there are illiterate people, you can explain BTC to them as in the concept of cash or barter. Meaning, BTCs can be exchanged for something like goods, services, etc. due to its significant value on the market. Additionally, you can also explain it to them that BTC works something as an investment- meaning you can store it for a relatively long time and it will incur value at the end (e.g. likening it to precious metals such as gold, etc.).

Even if there are illiterate people, this does not hinder them from learning about BTCs. More importantly, if they have the capacity to understand a complex currency such as BTC, then this can be their first step of learning how to read and write- thereby making them literate in the process.

In conclusion, teaching them about BTC can kill two birds in one stone- they learned about BTC; and they also have the opportunity to be literate at the same time.