[D7] Drupal.t() does not respect locale_custom_strings

I'm using $conf['locale_custom_strings_xx'] in settings.php and i have no problems with translations through t() php function. But if i translate through Drupal.t() javascript function there is no string translation from $conf['locale_custom_strings_xx']. Instead I have to use locales_target database table to translate strings through Drupal.t() javascript function.

Drupal.t() should also use $conf['locale_custom_strings_xx'].

Steps to reproduce

Easiest way to reproduce is:

1. enable second language (for example German)
2. specify some string overrides in settings.php, for example:

 $conf['locale_custom_strings_de'][''] = [
   'Label' => 'LabelCUSTOM',
   'Edit' => 'EditCUSTOM',
 ];

3. Go to /de/admin/structure/types/manage/page/fields
4. You can see the "Label" strings overriden correctly, but the "Edit" (when you start to type a new machine name) is not, as this is coming from Drupal.t() (instead it has "Bearbeiten" as a correct German translation).

(optionally)
5. apply patch
6. clear cache
7. "Bearbeiten" will be changed to "EditCUSTOM" correctly

Proposed resolution

Drupal.t() should respect the string overrides.