I also find it interesting to speculate on the nature of Petrie's mother tongue, if Leafeater, which we hear as English, is indeed a foreign language to him.
The way he pronounces English is unlike any broken English I've ever heard. English is a language with a rather simple grammar compared to other languages. But it has a high number of distinct vowels (14 to 18, depending on dialect), which is a rare characteristic among languages, and it has a peculiar way of pronouncing plosives (t, d) by putting the tongue on the gum, rather than on the upper teeth, as is done in most languages. Foreigners usually learn basic English grammar in no time, but they have difficulties pronouncing all the vowels and pulling back the tongue and rounding the lips and moving the cheek muscles the way natives do.
With Petrie it is the opposite. His pronounciation is like that of a native, but his grammar is atrocious. It seems like Petrie's mother tongue is a language of even simpler grammar than English, but of an even more complex phonology as well.
Petrie can, even with his creaky voice, apparently pronounce a tremendous range of sounds, but even the simplest grammatical structures present a challenge to him. It would be a language where sound changes are the utterly dominant way of expressing relations between words and the meanings of words, with no auxiliary verbs, adverbs, prepositions, prefixes and suffixes, and perhaps no syntax, to speak of.