Reset locale on login (#17734) (#18100)

Backport #17734

When logging in reset the user's locale to ensure that it matches their
preferred locale.

Fix #15612

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/web/user/auth.go

@@ -584,6 +584,10 @@ func handleSignInFull(ctx *context.Context, u *models.User, remember bool, obeyR
 
 	middleware.SetLocaleCookie(ctx.Resp, u.Language, 0)
 
+	if ctx.Locale.Language() != u.Language {
+		ctx.Locale = middleware.Locale(ctx.Resp, ctx.Req)
+	}
+
 	// Clear whatever CSRF has right now, force to generate a new one
 	middleware.DeleteCSRFCookie(ctx.Resp)