Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

modules/context/context.go

@@ -794,7 +794,7 @@ func Contexter() func(next http.Handler) http.Handler {
 			ctx.Data["UnitPullsGlobalDisabled"] = unit.TypePullRequests.UnitGlobalDisabled()
 			ctx.Data["UnitProjectsGlobalDisabled"] = unit.TypeProjects.UnitGlobalDisabled()
 
-			ctx.Data["i18n"] = locale
+			ctx.Data["locale"] = locale
 			ctx.Data["AllLangs"] = translation.AllLangs()
 
 			next.ServeHTTP(ctx.Resp, ctx.Req)