adapt README_{Country}.md stype name in localizedExtensions (#21486)

This stype name is also used in many repos, example:
[``README_ZH.md``](https://github.com/go-gitea/gitea/blob/main/README_ZH.md)

Signed-off-by: a1012112796 <1012112796@qq.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

routers/web/repo/view.go

@@ -151,8 +151,8 @@ func localizedExtensions(ext, languageCode string) (localizedExts []string) {
 	if strings.Contains(lowerLangCode, "-") {
 		underscoreLangCode := strings.ReplaceAll(lowerLangCode, "-", "_")
 		indexOfDash := strings.Index(lowerLangCode, "-")
-		// e.g. [.zh-cn.md, .zh_cn.md, .zh.md, .md]
-		return []string{lowerLangCode + ext, underscoreLangCode + ext, lowerLangCode[:indexOfDash] + ext, ext}
+		// e.g. [.zh-cn.md, .zh_cn.md, .zh.md, _zh.md, .md]
+		return []string{lowerLangCode + ext, underscoreLangCode + ext, lowerLangCode[:indexOfDash] + ext, "_" + lowerLangCode[1:indexOfDash] + ext, ext}
 	}
 
 	// e.g. [.en.md, .md]

routers/web/repo/view_test.go

@@ -38,19 +38,19 @@ func Test_localizedExtensions(t *testing.T) {
 			name:              "With region - lowercase",
 			languageCode:      "en-us",
 			ext:               ".md",
-			wantLocalizedExts: []string{".en-us.md", ".en_us.md", ".en.md", ".md"},
+			wantLocalizedExts: []string{".en-us.md", ".en_us.md", ".en.md", "_en.md", ".md"},
 		},
 		{
 			name:              "With region - uppercase",
 			languageCode:      "en-CA",
 			ext:               ".MD",
-			wantLocalizedExts: []string{".en-ca.MD", ".en_ca.MD", ".en.MD", ".MD"},
+			wantLocalizedExts: []string{".en-ca.MD", ".en_ca.MD", ".en.MD", "_en.MD", ".MD"},
 		},
 		{
 			name:              "With region - all uppercase",
 			languageCode:      "ZH-TW",
 			ext:               ".md",
-			wantLocalizedExts: []string{".zh-tw.md", ".zh_tw.md", ".zh.md", ".md"},
+			wantLocalizedExts: []string{".zh-tw.md", ".zh_tw.md", ".zh.md", "_zh.md", ".md"},
 		},
 	}
 	for _, tt := range tests {