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If I understand the question correctly... If he grew up speaking Old English (or whatever), is still alive, and communicates with present-day people, he will by definition have to have learned mod...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/8867 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If I understand the question correctly... If he grew up speaking Old English (or whatever), is still alive, and communicates with present-day people, he will by definition have to have learned modern English. If he can't speak modern English (or whatever the modern form of language is), then he won't be communicating with anyone. You can allude to the fact that his language evolved over time (all living languages change as time passes). If you don't write his dialogue in English (or whatever living language you're writing in), no one will be able to understand his dialogue. You can also have him use slightly stilted or idiosyncratic English to portray his different linguistic background.