A django application is in process of adding support for second language. During adding translations to the django.po file after next .po file compilation some of previously translated texts were displayed on target site in their original, untranslated form.
Afert usual digging in google’s search results the answer was found: a “#, fuzzy” flag was responsible for not translating my strings.
Fuzzy flag description. In short words: if you are sure that “fuzzy” marked translation is correct simply remove “fuzzy” flag, compile the .po file and you are done.