Great question — answering as the UphillTech team:
There are two GMS encoding schemes, GSM 7-bit (which you are seeing as text because it is decoded for you by the SIM800) and UCS-2, which represents the most commonly used Latin and eastern characters in these two-byte characters. The UCS-2 encodings are all identical to UTF-16 so you can decode using that. You know it's a UCS-2 message from the 'data coding scheme' entry in the SIM800 response to +CMGR. You can find out more by searching for 'GSM 03.38 character encoding'.
Full answer by Nick Rozanski: https://stackoverflow.stackexchange.com/a/62242604 (CC BY-SA 4.0).
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