Make better use of i18n (#20096)

* Prototyping

* Start work on creating offsets

* Modify tests

* Start prototyping with actual MPH

* Twiddle around

* Twiddle around comments

* Convert templates

* Fix external languages

* Fix latest translation

* Fix some test

* Tidy up code

* Use simple map

* go mod tidy

* Move back to data structure

- Uses less memory by creating for each language a map.

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* Add some comments

* Fix tests

* Try to fix tests

* Use en-US as defacto fallback

* Use correct slices

* refactor (#4)

* Remove TryTr, add log for missing translation key

* Refactor i18n

- Separate dev and production locale stores.
- Allow for live-reloading in dev mode.

Co-authored-by: zeripath <art27@cantab.net>

* Fix live-reloading & check for errors

* Make linter happy

* live-reload with periodic check (#5)

* Fix tests

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

templates/repo/issue/view_content/pull.tmpl

@@ -4,7 +4,7 @@
 			<div class="ui segment">
 				<h4>{{$.i18n.Tr "repo.issues.review.reviewers"}}</h4>
 				{{range .PullReviewers}}
-					{{ $createdStr:= TimeSinceUnix .Review.UpdatedUnix $.i18n.Lang }}
+					{{ $createdStr:= TimeSinceUnix .Review.UpdatedUnix $.i18n }}
 					<div class="ui divider"></div>
 					<div class="review-item">
 						<div class="review-item-left">
@@ -82,7 +82,7 @@
 					</div>
 				{{end}}
 				{{range .OriginalReviews}}
-					{{ $createdStr:= TimeSinceUnix .UpdatedUnix $.i18n.Lang }}
+					{{ $createdStr:= TimeSinceUnix .UpdatedUnix $.i18n }}
 					<div class="ui divider"></div>
 					<div class="review-item">
 						<div class="review-item-left">
@@ -328,7 +328,7 @@
 					{{if or $prUnit.PullRequestsConfig.AllowMerge $prUnit.PullRequestsConfig.AllowRebase $prUnit.PullRequestsConfig.AllowRebaseMerge $prUnit.PullRequestsConfig.AllowSquash}}
 						{{$hasPendingPullRequestMergeTip := ""}}
 						{{if .HasPendingPullRequestMerge}}
-							{{$createdPRMergeStr := TimeSinceUnix .PendingPullRequestMerge.CreatedUnix $.i18n.Lang}}
+							{{$createdPRMergeStr := TimeSinceUnix .PendingPullRequestMerge.CreatedUnix $.i18n}}
 							{{$hasPendingPullRequestMergeTip = $.i18n.Tr "repo.pulls.auto_merge_has_pending_schedule" .PendingPullRequestMerge.Doer.Name $createdPRMergeStr}}
 						{{end}}
 						<div class="ui divider"></div>