ui: encourage participation in the localization of Forgejo in language settings (#4596)

Changes:
- clarify how the language selector works as this is not very unclear: the footer selector is per-session and the settings selector is per-account, usually users need the 2nd
- tell about how to participate in improvement of localization via this link: https://forgejo.org/docs/latest/developer/localization/. Test that the link is present by extending and existing test

Preview:
- https://codeberg.org/attachments/f5d04fa1-2e9a-46e4-ac94-d8effefb9762
- https://codeberg.org/attachments/2a820bf5-1326-4c4a-9aff-4b9b70ee1bc0

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4596
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>

options/locale/locale_en-US.ini

@@ -741,6 +741,8 @@ continue = Continue
 cancel = Cancel
 language = Language
 language.title = Default language
+language.description = This language will be saved to your account and be used as the default after you log in.
+language.localization_project = Help us translate Forgejo into your language! <a href="%s">Learn more</a>.
 ui = Theme
 hints = Hints
 additional_repo_units_hint = Suggest to enable additional repository units

templates/user/settings/appearance.tmpl

@@ -47,6 +47,9 @@
 		</h4>
 		<div class="ui attached segment">
 			<form class="ui form" action="{{.Link}}/language" method="post">
+				<div class="tw-mb-4">
+					{{ctx.Locale.Tr "settings.language.description"}}
+				</div>
 				{{.CsrfTokenHtml}}
 				<div class="field">
 					<div class="ui language selection dropdown" id="language">
@@ -60,6 +63,9 @@
 						</div>
 					</div>
 				</div>
+				<div class="tw-mb-4">
+					{{ctx.Locale.Tr "settings.language.localization_project" "https://forgejo.org/docs/latest/developer/localization/"}}
+				</div>
 				<div class="field">
 					<button class="ui primary button">{{ctx.Locale.Tr "settings.update_language"}}</button>
 				</div>

tests/integration/user_test.go

@@ -380,6 +380,9 @@ func TestUserHints(t *testing.T) {
 
 			_, hintChecked := htmlDoc.Find(`input[name="enable_repo_unit_hints"]`).Attr("checked")
 			assert.Equal(t, enabled, hintChecked)
+
+			link, _ := htmlDoc.Find("form[action='/user/settings/appearance/language'] a").Attr("href")
+			assert.EqualValues(t, "https://forgejo.org/docs/latest/developer/localization/", link)
 		}
 
 		t.Run("view", func(t *testing.T) {