It's Bulgarian! And one of my favorite psalms! Thank you, Google Translator!
"Remember, Lord, King David /
and all his meekness, Hallelujah /
how he swore to the LORD /
'I will not give sleep to mine eyes /
till I find a place for the Lord.' Hallelujah."
The subtitles may be, but the singing is definitely Slavonic.
You could say with much more justification: "The subtitles may be and the singing is naturally Slavonic." From the Wiki:
"Old Church Slavonic, also known as Old Bulgarian, or Old Macedonian, was the first literary Slavic language, based on the old Slavic dialect of the Thessalonica region, employed by the 9th century Byzantine Greek missionaries, Saints Cyril and Methodius, who used it for translation of the Bible and other Ancient Greek ecclesiastical texts, and for some of their own writings. It played a great role in the history of Slavic languages and served as a basis and model for later Church Slavonic traditions, where Church Slavonic is used as a liturgical language to this day by some Orthodox and Greek-Catholic Churches of the Slavic peoples."
FYI: Services in most South and East Slavic Churches contain both the modern vernacular and Church Slavonic. Again from the Wiki: "Historically, this language is derived from Old Church Slavonic by adapting pronunciation and orthography and replacing some old and obscure words and expressions with their vernacular counterparts."
BTW, I am not familiar with the term "Slavonic singing." Do you instead mean "Slavic" singing? For example, the Bulgarian Church uses both Byzantine chanting/tones and tones and choral compositions from Slavic nations and and composers.