Dynamic default language for translation

Hello,

I am new here and also new to Drupal. Need your advice.

We have multilingual a site. The default language is English and most of the languages will be translated from English. However, we have language like Ukrainian which make sense to translate from Russian, so not translated pages will still remain in Russian. Most of Ukrainians can read Russian better than English; That may be also true for some Arabic languages with French as a second language.

Let me know if you know how to do Dynamic default language for translation.
Thank you.