In this case, I cannot agree that there is such a thing as pomak language.
In Bulgaria there is the official Bulgarian language, that was created in the late 19th century, based on a few dialects from the area of Veliko Tarnovo and Sofia regions and there is the spoken vernaculars.
Wherever you go in Bulgaria, all small villages have their own dialects based on the region.
These are dialects that have evolved for centuries, but which have one thing in common - the grammar is Bulgarian.
There exists a grammatical linguistic unity between all Bulgarian dialects. Often the dialects in the different regions differ in the morphology and the range of foreign words.
The western dialects, being transitional between Bulgarian and Serb languages, have a lot of Serbian words. The Southern dialects respectively have a lot of Turkish and Greek words.
What unites all these dialects is the grammar and the syntax - the way sentences are being built. There lies the difference between Bulgarian and other Slavic languages.
The pomak villages speak the dialects that are spoken in the region. It is a shared language between Christians and Muslims and that is why we cannot claim that pomak is different from Bulgarian.
Reasons:
1. Pomaks do not speak one language. We speak different languages, depending on what part of Bulgaria we live in. The Pomaks in Northern Bulgaria speak like the Christians there - northern Bulgarian dialects
The pomaks in Pirin region speak pirin dialects, just like their Christian neighbours - Pirin dialects.
Pomaks in Rhodopi speak same rhodopean dialect as the Christians there.
2. The dialects in the pomak villages employ Bulgarian grammar, morphology and syntax. There is a lot of Turkish words from the Ottoman era, but this is valid for all Bulgaria. Wherever you go, the dialects have a lot of Turkish words in them. The Burgas region populated mainly by Christians have tons of Turkish words in their vernacular. Varna region has tons of Turkish words etc., etc.
With the introduction of the modern Bulgarian language in the late 19th c. most of Bulgaria is trying to get rid of the foreign words and tries to introduce the Bulgarian equivalents instead.
3. We cannot claim that there is such a thing as pomak language, mainly because there is no unity between the different dialects spoken in pomak villages across Bulgaria. The underlying unity is Bulgarian syntax, morphology and grammar.
There is huge differences in the dialects across Bulgaria, but again these are all dialects of the Bulgarian group.
Southern Slavic languages include Serbian, Croatian, Slovenian, Bosnian, Bulgarian, and Macedonian. A lot of scholars assign Macedonians to Bulgarian or name it as transitional between BG and Serbian.
All dialects in these groups differ a lot, but they are all united by the linguistic characteristics of the respective group.
The dialects spoken in the pomak villages are of the Bulgarian group - heavily influenced by Turkish and with a lot of ancient Slavic remnants and archaisms, but nonetheless these are all Bulgarian dialects.
