A piece of information which I missed in the article: ‘yoghurt’ was an almost completely unknown word in Bulgaria until as soon as 2-3 decades ago. We call the substance „кисело млѣко“ (kiselo mlěko)[1] which literally means ‘sour milk’. Nowadays there are some products that are commercially available and are denoted as ‘yoghurt’ (Bulg. „йогурт“), but the word still seems somewhat unnatural for most Bulgarians and may not be completely understood by some people (e.g. older, from the countryside, etc).

I am not a specialist, but the products that we in English call ‘yoghurt’, ‘soured milk’, ‘buttermilk’, ‘kefir’ are somewhat related and as far as I can understand from my cursory research now, the former two fall under the formerly shown term in Bulgarian, while the latter two are often referred to as „мътен“ (măten), „мътеница“/„матеница“ (mătenitsa/matenitsa), „бутаница“ (butanitsa) and a few other regional names. Kefir is also often just called that way if it was imported, e.g. from Russia or somewhere in the Caucausus–as is usually done with other things (sometimes also with foreign-origin yoghurt). ‘strained yogurt’ is „цедено кисело мляко“ (lit. the same), and there are also other milk products some of which I am not entirely sure how to explain since I don't know how exactly they are made, e.g. „катък“ (katăk–something like a milk-based spread; essentially the same name as ‘qatiq’ in many Turkic-speaking regions, but AFAIK the same name can refer to relatively/quite different things from place to place), „таратор“ (‘tarator’–a cold soup, similar to ‘ayran’ with some specific added ingredients to it), „сух таратор“ (lit. ‘dry tarator’, similar to Gr. tzatziki and Tur. cacık) and a number of others which people who are better aware of the Bulgarian culinary traditions would've probably mentioned here.

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Following is my completely wild guess, but unfortunately I wasn't able to find reliable sources to either prove or disprove it, so take from it whatever you want. Will be glad if somebody could chip in with more knowledge.

The Bulgarian term for yoghurt (as already said also not unknown in other South Slavic languages, but the single most–and virtually universally–used term in our language) seems closely related to the old Greek type of yoghurt «οξύγαλα», which literally means the same (‘sour milk’). It was not uncommon for Bulgarians to translate terms from Greek during Medieval times, but they just took the Turkish word for all new things that came during Ottoman times, with the most prominent examples that remain nowadays all being foods (e.g. various plants or cooked dishes). Romanian features similar ways to refer to yogurt (although ‘iaurt’ seems more common nowadays), but I don't know how many of them are just calques, regional variants, or what the origin of each is in particular. What is more, Albanian also has its own–although seemingly unrelated–words for yogurt, buttermilk and various other milk products. This leads me to think that ‘yoghurt’ was one of the things that was known on the Balkans long before the Ottoman conquest and the Slavic speakers back then knew it–or something essentially similar–quite well (in stark contrast to another beloved milk product made out of yogurt: „айран“, the name of which comes directly from the Turkish ‘ayran’).

P.S. As of now the Wikipedia article on the bacteria mentioned in the article[2] says the following: “Lactobacillus delbrueckii subsp. bulgaricus can be found naturally in the gastrointestinal tract of mammals living in Bulgaria, but one specific strain, Lactobacillus bulgaricus GLB44, is extracted from the leaves of the Galanthus nivalis (snowdrop flower) in Bulgaria as well. The bacterium is also grown artificially in many other countries.” It is yet to be given a proper citation, but this is the anecdotal information I have heard on numerous occasions circulated in Bulgaria as the source for the naming as opposed to the country of origin of the person who discovered it. I don't know which version–if any–is correct, though.

P.P.S. There are also other milk products in Bulgaria with traditional names:

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[1] Official writing nowadays is „кисело мляко“, but I have left the former spelling on purpose because this is etymologically more informative and makes a more visible connection to all Bulgarian dialects as well as to all modern South Slavic languages (also the ones from the western branch), where ‘kiselo mlijeko’ is also not unknown.

[2] https://en.wikipedia.org/wiki/Lactobacillus_delbrueckii_subs...