… There are probably some languages which don’t have a name for themselves, and some languages which don’t have written representations, but apart from these, one could reasonably denote a language by writing the name of the language in that language’s most common written form. After all, we have Unicode.

English / español / Sermo Vulgaris / Deutsch / le français / kreyòl ayisyen / 官话 / 日本語 /   …

“U+016080 to U+0160FF in the SMP is tentatively allocated for tengwar”, says Wikipedia.