One could follow the example of C11. 6.4.2.1 General, n.71 suggests
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On systems in which linkers cannot accept extended characters, an
encoding of the universal character name may be used in forming valid
external identifiers. For example, some otherwise unused character or
sequence of characters may be used to encode the \u in a universal
character name. Extended characters may produce a long external
identifier
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However for consistency, having a special mangle/encoding of UTF8 characters in symbol names could be done instead.
Since UTF8 characters in symbols are not link compatible with C or C++ anyway, there is no problem in doing it this way across all linkages.