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

@@ -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

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