Your langcode isn't standard, you can associate a standard langcode here.

I have error message

Your langcode isn't standard, you can associate a standard langcode here.

But my language is quite standard: ru.

ksm(checkStandardLanguageCode('ru'));

$formulae = \Drupal::state()->get('locale.translation.formulae', []);
ksm($formulae);

returns

boolean true

array (1)
ru => array (2)
plurals => string (1) "3"
formula => array (73)