How do you likely to say for non-English speaker to use letters from the English alphabet (e.g. to write down something or to create a nickname)?
Do you say use Latin letters or English letters?
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The technical term for the alphabet used to write English is Latin alphabet. The English alphabet, technically, would be the one used to write Old and Middle English, which contained some additional letters like the thorn (þ) and eth (Ð). Those letters fell out of use with the rise of the printing press, which normally only had the Latin alphabet available.
An alternative for technical uses, like selecting a user name, would be alphanumeric characters. This refers to the letters of the Latin alphabet and Arabic numerals.
You could say
Please use the letters A to Z (from the Latin alphabet).
If you need to specify a code-point range to be very precise, specify the codepoint range in Unicode, or you could refer to the letters "in the ASCII range".