diff --git a/core/artwork.go b/core/artwork.go
index 59df626..7ac6b9d 100644
--- a/core/artwork.go
+++ b/core/artwork.go
@@ -29,14 +29,14 @@ const (
 	R18G XRestrict = 2
 )
 
-func (x XRestrict) Localize(l i18n.Localizer) string {
+func (x XRestrict) String() string {
 	switch x {
 	case Safe:
-		return l.Tr("Safe")
+		return i18n.Tr("Safe")
 	case R18:
-		return l.Tr("R18")
+		return i18n.Tr("R18")
 	case R18G:
-		return l.Tr("R18G")
+		return i18n.Tr("R18G")
 	}
 	log.Panicf("invalid value: %#v", int(x))
 	return ""
@@ -56,14 +56,14 @@ const (
 	AI      AiType = 2
 )
 
-func (x AiType) Localize(l i18n.Localizer) string {
+func (x AiType) String() string {
 	switch x {
 	case Unrated:
-		return l.Tr("Unrated")
+		return i18n.Tr("Unrated")
 	case NotAI:
-		return l.Tr("Not AI")
+		return i18n.Tr("Not AI")
 	case AI:
-		return l.Tr("AI")
+		return i18n.Tr("AI")
 	}
 	log.Panicf("invalid value: %#v", int(x))
 	return ""
diff --git a/doc/dev/features/i18n.md b/doc/dev/features/i18n.md
index f9850f5..b5b4ca9 100644
--- a/doc/dev/features/i18n.md
+++ b/doc/dev/features/i18n.md
@@ -1,10 +1,11 @@
 # Localization
 
-Notes on i18n/
+Important notice: please don't translate server logs with `i18n.*()`. It uses the request locale, not the server one.
 
 ## Todo
 
-- [ ] Add env PIXIVFE_SERVER_LOCALE and set `i18n.GlobalLocale` to that on startup
+- [x] Add cookie: Locale
+- [ ] Add option to set locale in settings page
 - [ ] Rewrite jet template on load to use translated strings
 - [ ] Check if any jet template strings are ignored (false negative)
 - Setup Crowdin
diff --git a/go.mod b/go.mod
index 153cf2b..c7fa20b 100644
--- a/go.mod
+++ b/go.mod
@@ -21,6 +21,7 @@ require (
 	github.com/sethvargo/go-limiter v1.0.0
 	github.com/soluble-ai/go-jnode v0.1.11
 	github.com/tidwall/gjson v1.18.0
+	github.com/timandy/routine v1.1.4
 	github.com/yargevad/filepathx v1.0.0
 	github.com/zeebo/xxh3 v1.0.2
 	golang.org/x/net v0.30.0
diff --git a/go.sum b/go.sum
index c93120f..b79b705 100644
--- a/go.sum
+++ b/go.sum
@@ -10,6 +10,8 @@ github.com/PuerkitoBio/urlesc v0.0.0-20170810143723-de5bf2ad4578 h1:d+Bc7a5rLufV
 github.com/PuerkitoBio/urlesc v0.0.0-20170810143723-de5bf2ad4578/go.mod h1:uGdkoq3SwY9Y+13GIhn11/XLaGBb4BfwItxLd5jeuXE=
 github.com/andybalholm/cascadia v1.3.2 h1:3Xi6Dw5lHF15JtdcmAHD3i1+T8plmv7BQ/nsViSLyss=
 github.com/andybalholm/cascadia v1.3.2/go.mod h1:7gtRlve5FxPPgIgX36uWBX58OdBsSS6lUvCFb+h7KvU=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/fatih/color v1.16.0 h1:zmkK9Ngbjj+K0yRhTVONQh1p/HknKYSlNT+vZCzyokM=
 github.com/fatih/color v1.16.0/go.mod h1:fL2Sau1YI5c0pdGEVCbKQbLXB6edEj1ZgiY4NijnWvE=
 github.com/go-faker/faker/v4 v4.5.0 h1:ARzAY2XoOL9tOUK+KSecUQzyXQsUaZHefjyF8x6YFHc=
@@ -37,18 +39,24 @@ github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D
 github.com/oklog/ulid/v2 v2.1.0 h1:+9lhoxAP56we25tyYETBBY1YLA2SaoLvUFgrP2miPJU=
 github.com/oklog/ulid/v2 v2.1.0/go.mod h1:rcEKHmBBKfef9DhnvX7y1HZBYxjXb0cP5ExxNsTT1QQ=
 github.com/pborman/getopt v0.0.0-20170112200414-7148bc3a4c30/go.mod h1:85jBQOZwpVEaDAr341tbn15RS4fCAsIst0qp7i8ex1o=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/sethvargo/go-envconfig v1.1.0 h1:cWZiJxeTm7AlCvzGXrEXaSTCNgip5oJepekh/BOQuog=
 github.com/sethvargo/go-envconfig v1.1.0/go.mod h1:JLd0KFWQYzyENqnEPWWZ49i4vzZo/6nRidxI8YvGiHw=
 github.com/sethvargo/go-limiter v1.0.0 h1:JqW13eWEMn0VFv86OKn8wiYJY/m250WoXdrjRV0kLe4=
 github.com/sethvargo/go-limiter v1.0.0/go.mod h1:01b6tW25Ap+MeLYBuD4aHunMrJoNO5PVUFdS9rac3II=
 github.com/soluble-ai/go-jnode v0.1.11 h1:To9h6zWI6qr5Vphz0XO4JtCRlkhfilH2EDNwtAZkZT4=
 github.com/soluble-ai/go-jnode v0.1.11/go.mod h1:mKFUM7xxXMBVOygZeE8od2Qls1f3E3NHY1ZhMlzwx/U=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 github.com/tidwall/gjson v1.18.0 h1:FIDeeyB800efLX89e5a8Y0BNH+LOngJyGrIWxG2FKQY=
 github.com/tidwall/gjson v1.18.0/go.mod h1:/wbyibRr2FHMks5tjHJ5F8dMZh3AcwJEMf5vlfC0lxk=
 github.com/tidwall/match v1.1.1 h1:+Ho715JplO36QYgwN9PGYNhgZvoUSc9X2c80KVTi+GA=
 github.com/tidwall/match v1.1.1/go.mod h1:eRSPERbgtNPcGhD8UCthc6PmLEQXEWd3PRB5JTxsfmM=
 github.com/tidwall/pretty v1.2.0 h1:RWIZEg2iJ8/g6fDDYzMpobmaoGh5OLl4AXtGUGPcqCs=
 github.com/tidwall/pretty v1.2.0/go.mod h1:ITEVvHYasfjBbM0u2Pg8T2nJnzm8xPwvNhhsoaGGjNU=
+github.com/timandy/routine v1.1.4 h1:L9eAli/ROJcW6LhmwZcusYQcdAqxAXGOQhEXLQSNWOA=
+github.com/timandy/routine v1.1.4/go.mod h1:siBcl8iIsGmhLCajRGRcy7Y7FVcicNXkr97JODdt9fc=
 github.com/yargevad/filepathx v1.0.0 h1:SYcT+N3tYGi+NvazubCNlvgIPbzAk7i7y2dwg3I5FYc=
 github.com/yargevad/filepathx v1.0.0/go.mod h1:BprfX/gpYNJHJfc35GjRRpVcwWXS89gGulUIU5tK3tA=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
@@ -97,3 +105,5 @@ golang.org/x/tools v0.0.0-20191203134012-c197fd4bf371/go.mod h1:b+2E5dAYhXwXZwtn
 golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
 golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/i18n/global.go b/i18n/global.go
index d470c7c..cb73d50 100644
--- a/i18n/global.go
+++ b/i18n/global.go
@@ -3,26 +3,36 @@ package i18n
 import (
 	"errors"
 	"fmt"
+
+	"github.com/timandy/routine"
 )
 
-var GlobalLocale string = "en"
+var Locale = routine.NewInheritableThreadLocal[string]()
+
+func getLocalizer() Localizer {
+	locale := Locale.Get()
+	if locale == "" {
+		locale = "en"
+	}
+	return LocalizerOf(locale)
+}
 
 func Error(text string) error {
-	text = LocalizerOf(GlobalLocale).lookup(text)
+	text = getLocalizer().lookup(text)
 	return errors.New(text)
 }
 
 func Errorf(format string, a ...any) error {
-	format = LocalizerOf(GlobalLocale).lookup(format)
+	format = getLocalizer().lookup(format)
 	return fmt.Errorf(format, a...)
 }
 
 func Sprintf(format string, a ...any) string {
-	format = LocalizerOf(GlobalLocale).lookup(format)
+	format = getLocalizer().lookup(format)
 	return fmt.Sprintf(format, a...)
 }
 
 // translate string
 func Tr(text string) string {
-	return LocalizerOf(GlobalLocale).lookup(text)
+	return getLocalizer().lookup(text)
 }
diff --git a/i18n/lookup.go b/i18n/lookup.go
index 5772a8a..49e3f70 100644
--- a/i18n/lookup.go
+++ b/i18n/lookup.go
@@ -22,7 +22,7 @@ func LocalizerOf(locale string) Localizer {
 	return Localizer{locale: locale}
 }
 
-// translate string
+// lookup string in the database
 func (l Localizer) lookup(text string) string {
 	translation_map, exist := locales[l.locale]
 	if !exist {
diff --git a/main.go b/main.go
index 205f9b6..9c250e0 100644
--- a/main.go
+++ b/main.go
@@ -30,28 +30,30 @@ func main() {
 
 	// Conditionally initialize and start the proxy checker
 	if config.GlobalConfig.ProxyCheckEnabled {
-		var wg_firstCheck sync.WaitGroup
-		wg_firstCheck.Add(1)
-
 		go func() {
-			ctx, cancel := context.WithTimeout(context.Background(), config.GlobalConfig.ProxyCheckTimeout)
-			proxy_checker.CheckProxies(ctx)
-			cancel()
-			wg_firstCheck.Done()
-			if config.GlobalConfig.ProxyCheckInterval != 0 {
-				for {
-					time.Sleep(config.GlobalConfig.ProxyCheckInterval)
-					ctx, cancel := context.WithTimeout(context.Background(), config.GlobalConfig.ProxyCheckTimeout)
-					proxy_checker.CheckProxies(ctx)
-					cancel()
-				}
-			}
-		}()
+			var wg_firstCheck sync.WaitGroup
+			wg_firstCheck.Add(1)
 
-		// Wait for the first proxy check to complete
-		log.Println("Waiting for initial proxy check to complete...")
-		wg_firstCheck.Wait()
-		log.Println("Initial proxy check completed.")
+			go func() {
+				ctx, cancel := context.WithTimeout(context.Background(), config.GlobalConfig.ProxyCheckTimeout)
+				proxy_checker.CheckProxies(ctx)
+				cancel()
+				wg_firstCheck.Done()
+				if config.GlobalConfig.ProxyCheckInterval != 0 {
+					for {
+						time.Sleep(config.GlobalConfig.ProxyCheckInterval)
+						ctx, cancel := context.WithTimeout(context.Background(), config.GlobalConfig.ProxyCheckTimeout)
+						proxy_checker.CheckProxies(ctx)
+						cancel()
+					}
+				}
+			}()
+
+			// Wait for the first proxy check to complete
+			log.Println("Waiting for initial proxy check to complete...")
+			wg_firstCheck.Wait()
+			log.Println("Initial proxy check completed.")
+		}()
 	} else {
 		log.Println("Skipping proxy checker initialization.")
 	}
@@ -60,10 +62,11 @@ func main() {
 
 	router := middleware.DefineRoutes()
 	// the first middleware is the most outer / first executed one
-	router.Use(middleware.ProvideUserContext) // needed for everything else
-	router.Use(middleware.LogRequest)         // all pages need this
-	router.Use(middleware.SetPrivacyHeaders)  // all pages need this
-	router.Use(middleware.HandleError)        // if the inner handler fails, this shows the error page instead
+	router.Use(middleware.ProvideUserContext)  // needed for everything else
+	router.Use(middleware.SetLocaleFromCookie) // needed for i18n.*()
+	router.Use(middleware.LogRequest)          // all pages need this
+	router.Use(middleware.SetPrivacyHeaders)   // all pages need this
+	router.Use(middleware.HandleError)         // if the inner handler fails, this shows the error page instead
 	router.Use(middleware.InitializeRateLimiter())
 
 	// watch and compile sass when in development mode
diff --git a/server/middleware/set_locale.go b/server/middleware/set_locale.go
new file mode 100644
index 0000000..e85544b
--- /dev/null
+++ b/server/middleware/set_locale.go
@@ -0,0 +1,17 @@
+package middleware
+
+import (
+	"net/http"
+
+	"codeberg.org/vnpower/pixivfe/v2/i18n"
+	"codeberg.org/vnpower/pixivfe/v2/server/session"
+)
+
+// ProvideUserContext is a middleware that wraps an http.Handler
+// to inject a user context into each incoming request.
+func SetLocaleFromCookie(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		i18n.Locale.Set(session.GetCookie(r, session.Cookie_Locale))
+		h.ServeHTTP(w, r)
+	})
+}
diff --git a/server/session/cookie.go b/server/session/cookie.go
index 302e04e..e8f8643 100644
--- a/server/session/cookie.go
+++ b/server/session/cookie.go
@@ -21,6 +21,7 @@ const ( // the __Host thing force it to be secure and same-origin (no subdomain)
 	Cookie_HideArtR18        CookieName = "pixivfe-HideArtR18"
 	Cookie_HideArtR18G       CookieName = "pixivfe-HideArtR18G"
 	Cookie_HideArtAI         CookieName = "pixivfe-HideArtAI"
+	Cookie_Locale            CookieName = "pixivfe-Locale"
 )
 
 // Go can't make this a const...
@@ -35,6 +36,7 @@ var AllCookieNames []CookieName = []CookieName{
 	Cookie_HideArtR18,
 	Cookie_HideArtR18G,
 	Cookie_HideArtAI,
+	Cookie_Locale,
 }
 
 func GetCookie(r *http.Request, name CookieName) string {