TL;DR

• 動いてくれ頼む

Log

➜  ~ brew cask search firefox
==> Exact Match
firefox
==> Partial Matches
firefox-beta firefox-esr firefoxdeveloperedition firefoxnightly multifirefox
➜  ~
➜  ~ brew cask install firefoxnightly --language=en
Error: invalid option: --language=en

Refs

• https://rcmdnk.com/blog/2016/10/31/computer-mac-homebrew-firefox/
• https://github.com/Homebrew/homebrew-cask/blob/master/doc/cask_language_reference/stanzas/language.md#installation

追記1 [2017/06/25]

• 修正リリースを待つ
  • homebrew-cask/issues/35448