I don’t understand if there is still a community-agreed standard for such a country code.
However, for your sentence “more letters for the language or country code” this would certainly be a bad idea. The ISO standard has already determined how many characters can be a country / language code. For example, ISO 3166-1 alpha-3 is the long country code <char, and ISO 3166-1 alpha-2 is 2-char long.
I think that it’s best for you to choose a code that is not used by any country, or to choose from some remote codes so that no one uses or uses it. (For example, MID is a remote code that is well suited for averaging in Middle Earth).
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