FirefoxNightly: Use system language.

Casks/firefoxnightly-ja.rb

@@ -1,16 +0,0 @@
-cask 'firefoxnightly-ja' do
-  version '51.0a1'
-  sha256 '7b500b7526bc73f0f18ff7a6560a32dacf7b071156ef4046684c0c97f20be814'
-
-  url "https://ftp.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/firefox-#{version}.ja-JP-mac.mac.dmg"
-  name 'Mozilla Firefox'
-  homepage 'https://nightly.mozilla.org/'
-  license :mpl
-
-  app 'FirefoxNightly.app'
-
-  zap delete: [
-                '~/Library/Application Support/Firefox',
-                '~/Library/Caches/Firefox',
-              ]
-end

Casks/firefoxnightly-ru.rb

@@ -1,16 +0,0 @@
-cask 'firefoxnightly-ru' do
-  version '51.0a1'
-  sha256 :no_check # required as upstream package is updated in-place
-
-  url "https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/firefox-#{version}.ru.mac.dmg"
-  name 'Mozilla Firefox'
-  homepage 'https://nightly.mozilla.org/'
-  license :mpl
-
-  app 'FirefoxNightly.app'
-
-  zap delete: [
-                '~/Library/Application Support/Firefox',
-                '~/Library/Caches/Firefox',
-              ]
-end

Casks/firefoxnightly-uk.rb

@@ -1,16 +0,0 @@
-cask 'firefoxnightly-uk' do
-  version '51.0a1'
-  sha256 :no_check # required as upstream package is updated in-place
-
-  url "https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/firefox-#{version}.uk.mac.dmg"
-  name 'Mozilla Firefox'
-  homepage 'https://nightly.mozilla.org/'
-  license :mpl
-
-  app 'FirefoxNightly.app'
-
-  zap delete: [
-                '~/Library/Application Support/Firefox',
-                '~/Library/Caches/Firefox',
-              ]
-end

Casks/firefoxnightly.rb

@@ -2,7 +2,23 @@ cask 'firefoxnightly' do
   version '52.0a1'
   sha256 :no_check # required as upstream package is updated in-place
 
-  url "https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-#{version}.en-US.mac.dmg"
+  language 'en', default: true do
+    'en-US'
+  end
+
+  language 'ja' do
+    'ja-JP-mac'
+  end
+
+  language 'ru' do
+    'ru'
+  end
+
+  language 'uk' do
+    'uk'
+  end
+
+  url "https://ftp.mozilla.org/pub/firefox/nightly/latest-mozilla-central#{language == 'en-US' ? '' : '-l10n'}/firefox-#{version}.#{language}.mac.dmg"
   name 'Mozilla Firefox'
   homepage 'https://nightly.mozilla.org/'
   license :mpl