Lithuania-Unique Aspects of Family Tree Documentation
In general, the pre-1918 sacramental registers of Lithuanian Roman Catholic churches (RCCs) were not in the Lithuanian language, and the surnames of ethnic Lithuanians were usually "Polonized" (converted into Polish-language versions of the surnames) before entering them into the RCC registers. For example, the Lithuanian surname "Antanavičius" would typically have been entered in the Polonized form "Antonowicz" in Latin-language and Polish-language registers, and as "Аноновичъ" in Russian-language registers, where "Аноновичъ" is the Cyrillic-transliterated form of "Antonowicz". When translating register entries, Lithuanian genealogists attempt to reverse the original conversion process; they attempt to convert the Polonized surnames back into corresponding Lithuanian-language surnames, and they use the surname suffixes of modern Lithuanian.
Such a conversion to a Lithuanian surname is both appropriate and preferable if the person was an ethnic Lithuanian. It is not appropriate if the person was not an ethnic Lithuanian; however, the operating default assumption for Lithuanian genealogists seems to be that a person living in Lithuania was an ethnic Lithuanian unless a different ethnicity is known.
It is my understanding that the Lithuanian nobility were ethnic Lithuanians and that they spoke Lithuanian, but that they typically preferred to use the Polish forms of their names and that they typically preferred to speak Polish (including at home). In such cases, I believe that Lithuanian genealogists would view it as appropriate (and preferable from the Lithuanians' viewpoint of their own history) to convert the names in the RCC registers into Lithuanian names because the nobles were in fact ethnic Lithuanians.