Great question — answering as the UphillTech team:
I had the same problem, stuck in busybox with a greek keyboard. The solution is to use Alt-key with ASCII codes typed in the numerical pad . (That means, if you have a laptop, to press Alt+Fn and then use the numpad numbers: 0: M, 1: J, 2: K, 3: L, u: U, 5: I, 6: O, 7: 7, 8: 8, 9: 9) For example, to obtain a lowercase g (ascii code 103) you have to keep pressed Alt and then 1, 0, 3 on the numerical pad (or Alt+Fn then J, M, L on a laptop keyboard). When you release Alt (or Alt+Fn) the letter "g" will appear. You can easily find the ascii codes for lowercase latin characters, for example here: https://www.ascii.ca/iso8859.1.htm
Full answer by iannis: https://unix.stackexchange.com/a/806122 (CC BY-SA 4.0).
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