Extracting i18n functionality to i18n core library (#16914)

* extracting i18n functionality to i18n core library

* Removing utils.T

* Adding documentation and changing one function name for better explanation

* Changing other missing utils.T

* Adding license string

* Renaming corelibs to pkg

* Renaming corelibs to pkg (moving directory)

* Renaming from pkg to shared

* Fixing bodyPage.Html casing

* Fixing merges

* Fixing merge problem

* Fixing tests

app/server.go

@@ -54,6 +54,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/services/timezones"
 	"github.com/mattermost/mattermost-server/v5/services/tracing"
 	"github.com/mattermost/mattermost-server/v5/services/upgrader"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/mattermost/mattermost-server/v5/store/localcachelayer"
 	"github.com/mattermost/mattermost-server/v5/store/retrylayer"
@@ -288,7 +289,7 @@ func NewServer(options ...Option) (*Server, error) {
 	if err := utils.TranslationsPreInit(); err != nil {
 		return nil, errors.Wrapf(err, "unable to load Mattermost translation files")
 	}
-	model.AppErrorInit(utils.T)
+	model.AppErrorInit(i18n.T)
 
 	searchEngine := searchengine.NewBroker(s.Config(), s.Jobs)
 	bleveEngine := bleveengine.NewBleveEngine(s.Config(), s.Jobs)
@@ -328,7 +329,7 @@ func NewServer(options ...Option) (*Server, error) {
 
 	s.createPushNotificationsHub()
 
-	if err2 := utils.InitTranslations(s.Config().LocalizationSettings); err2 != nil {
+	if err2 := i18n.InitTranslations(*s.Config().LocalizationSettings.DefaultServerLocale, *s.Config().LocalizationSettings.DefaultClientLocale); err2 != nil {
 		return nil, errors.Wrapf(err2, "unable to load Mattermost translation files")
 	}
 
@@ -1043,7 +1044,7 @@ func (s *Server) Start() error {
 
 	listener, err := net.Listen("tcp", addr)
 	if err != nil {
-		return errors.Wrapf(err, utils.T("api.server.start_server.starting.critical"), err)
+		return errors.Wrapf(err, i18n.T("api.server.start_server.starting.critical"), err)
 	}
 	s.ListenAddr = listener.Addr().(*net.TCPAddr)
 
@@ -1059,7 +1060,7 @@ func (s *Server) Start() error {
 		if host, port, err := net.SplitHostPort(addr); err != nil {
 			mlog.Error("Unable to setup forwarding", mlog.Err(err))
 		} else if port != "443" {
-			return fmt.Errorf(utils.T("api.server.start_server.forward80to443.enabled_but_listening_on_wrong_port"), port)
+			return fmt.Errorf(i18n.T("api.server.start_server.forward80to443.enabled_but_listening_on_wrong_port"), port)
 		} else {
 			httpListenAddress := net.JoinHostPort(host, "http")
 
@@ -1088,7 +1089,7 @@ func (s *Server) Start() error {
 			}
 		}
 	} else if *s.Config().ServiceSettings.UseLetsEncrypt {
-		return errors.New(utils.T("api.server.start_server.forward80to443.disabled_while_using_lets_encrypt"))
+		return errors.New(i18n.T("api.server.start_server.forward80to443.disabled_while_using_lets_encrypt"))
 	}
 
 	s.didFinishListen = make(chan struct{})
@@ -1183,15 +1184,15 @@ func (s *Server) startLocalModeServer() error {
 
 	socket := *s.configStore.Get().ServiceSettings.LocalModeSocketLocation
 	if err := os.RemoveAll(socket); err != nil {
-		return errors.Wrapf(err, utils.T("api.server.start_server.starting.critical"), err)
+		return errors.Wrapf(err, i18n.T("api.server.start_server.starting.critical"), err)
 	}
 
 	unixListener, err := net.Listen("unix", socket)
 	if err != nil {
-		return errors.Wrapf(err, utils.T("api.server.start_server.starting.critical"), err)
+		return errors.Wrapf(err, i18n.T("api.server.start_server.starting.critical"), err)
 	}
 	if err = os.Chmod(socket, 0600); err != nil {
-		return errors.Wrapf(err, utils.T("api.server.start_server.starting.critical"), err)
+		return errors.Wrapf(err, i18n.T("api.server.start_server.starting.critical"), err)
 	}
 
 	go func() {