From 143a824449e86e603ef0ccfdbfc705d349dc6b16 Mon Sep 17 00:00:00 2001
From: Natalie Tay <natalie.tay@gmail.com>
Date: Wed, 19 Feb 2025 11:55:14 +0800
Subject: [PATCH] FIX: Language parser matches with dashes or underscores
 (#31381)

Our language parser now incorrectly matches underscored locales:

```
[1] pry(main)> HttpLanguageParser.parse("zh-CN")
=> "zh_CN"
[2] pry(main)> HttpLanguageParser.parse("zh_CN")
=> "en_GB"
```

This commit makes sure the input can be agnostic of `-` or `_`
---
 lib/http_language_parser.rb           |  2 +-
 spec/lib/http_language_parser_spec.rb | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 spec/lib/http_language_parser_spec.rb

diff --git a/lib/http_language_parser.rb b/lib/http_language_parser.rb
index a2e24ca0aae..cf83236ce67 100644
--- a/lib/http_language_parser.rb
+++ b/lib/http_language_parser.rb
@@ -6,7 +6,7 @@ module HttpLanguageParser
     # headers use hyphens.
     require "http_accept_language" unless defined?(HttpAcceptLanguage)
     available_locales = I18n.available_locales.map { |locale| locale.to_s.tr("_", "-") }
-    parser = HttpAcceptLanguage::Parser.new(header)
+    parser = HttpAcceptLanguage::Parser.new(header&.tr("_", "-"))
     matched = parser.language_region_compatible_from(available_locales)&.tr("-", "_")
     matched || SiteSetting.default_locale
   end
diff --git a/spec/lib/http_language_parser_spec.rb b/spec/lib/http_language_parser_spec.rb
new file mode 100644
index 00000000000..ba75fc6ba55
--- /dev/null
+++ b/spec/lib/http_language_parser_spec.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+describe HttpLanguageParser do
+  it "returns the default locale when no language is matched" do
+    expect(HttpLanguageParser.parse("")).to eq(SiteSetting.default_locale)
+  end
+
+  it "returns the matched locale when a language is matched" do
+    expect(HttpLanguageParser.parse("en")).to eq("en")
+  end
+
+  it "returns the matched locale when a language and region are matched" do
+    expect(HttpLanguageParser.parse("en-US")).to eq("en")
+  end
+
+  it "returns the matched locale regardless of dash or underscore usage" do
+    expect(HttpLanguageParser.parse("zh-CN")).to eq("zh_CN")
+    expect(HttpLanguageParser.parse("zh_CN")).to eq("zh_CN")
+  end
+end