Koji Fox is saying they are using old English words which examples have been posted here in this thread. If you read the thread you can see that along with some people saying Shaekespere.

"ホルムギャング" is written in Katakana here and phonetically spells out and sounds just like Holmgang "Ho Ru Mu Gya N Gu" It's supposed to directly sound like Holmgang as there is no Japanese translation in Kanji or Hiragana because it's not a word native to Japan. Katakana is used for foreign words outside of Japanese origin and Hiragana and Kanji are used for words native to Japan.