Now try to make a Japanese person to pronounce and spell it.
The spelling and pronounciation error might be becuase of difference in languages. Astlanian language is not clearly defined in the book but maybe they don’t have a sound or a letter that correspond to the English language. Think of the various languages and writing just here. In speach for example there might be a limitation of sound. Rotokas for example only has 11 phonemes (English has about 40). Writing is even more diverse. Try reading a German text. What are those weid letters?
Though as I can see. Astlanian language has similar phoneme set as English but maybe lack the ‘TH’ phoneme that might make them replace it with ‘T’ when they speak and write it. Astlanian also seems to be writing in alphabetic like English but maybe they don’t spell words with -TH- or use the letter(s) that correspond to the phoneme (like θ) in their writing which might also cause them to misspell Thomas’ name. It might even be not possible to spell his name right if Astlanians use Semanto-phonetic (like Chinese and Ancient Egyptian) or Sylabic (like Hiragana/Katakana Japanese and Thai). :-k