Exodus - English after 43k+ years

So presumably all human characters in the book and even celestials speak English. Or Chinese, or French - is not uncovered in the book - but some single language in general. Of course, I still assume there are tribes, planets, etc with local languages and dialects. But in general characters communicate with each other in the same language.

So. How the hell, after 43k real years in Centauri Cluster (500+ years Dilligent's timescale, dilated) Finn was able to talk to Ellie and Jossias right away in "English"???. How the hell, everyone talk to everyone in the same language (and understand each other), even in the most distant systems of Centrauri Cluster? (not even talking about Awakenings)

Excuse my own English. Not native. And just finished reading book1 - in English - took almost 2 months (but had lots of missed days).