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>

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) {