The key issue is really whether Unicode supports the language, rather than vice versa.
There are languages that aren't in Unicode (yet), but they are generally relatively obscure dead languages.
Also, keep in mind that Unicode continues to expand, so if there were some language unsupported today, it might be supported in a future version of Unicode.
As OpenType allows encoding of any Unicode codepoint, even ones that are not yet assigned, it should be able to handle even future versions of Unicode.
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