A chat exported from an Azerbaijani phone is a line like this: 12.10.2026, 14:05 - Əli: salam. The file is UTF-8. When you double-click it, Excel opens it with the system table. That table has no ə, so the letter becomes ? or another mark.
Do not open the .txt straight in Excel. Write a CSV first and put a BOM at the start.