(EU) 2016/679 (GDPR) is available in 24 languages. It wouldn't be practical for everyone communicating with EU citizens to be obliged to translate to all of them.
Article 12 in Chapter 3 section 1 specifies :
The controller shall take appropriate measures to provide any information referred to in Articles 13 and 14 and any communication under Articles 15 to 22 and 34 relating to processing to the data subject in a concise, transparent, intelligible and easily accessible form, using clear and plain language
If your application is available only in Ukranian and only people who understand Ukranian will be able to use it, your policies and responses to customer enquiries should also be in Ukrainian - but in a form that's "concise, transparent and easily accessible".