Это можно понять так:1) славяне понимали свою тартарскую часть територии,значит язык или идентичен или похож; 2)турки = тюрки = татары из одной компании исторически, доказывается в конце цитаты;
3)турки говорят по-славянски - такой вывод мог сделать любой иностранец, глядя на это всё со стороны (скорее всего, приходя через российскую территорию в Турцию).
Гил Флеччер 1588г., подданый Англии, путешественник и посол:


Their speech is verie sudden and loude, speaking as it were out of a deepe hollowe throate. When they sing you would thinke a kowe lowed, or some great bandogge howled. Their greatest exercise is shooting, wherein they traine vp their children from their verie infancie, not suffering them to eate till they haue shot neere the marke within a certaine scantling. They are the very same that sometimes were called Scythæ Nomades, or the Scythian shepheards, by the Greekes and Latines. Some thinke that the Turks took their beginning from the nation of the Crim Tartars. Of which opinion is Laonicus Calcocondylas the Greek Historiographer, in his first booke of his Turkish storie. Wherein hee followeth diuers verie probable coniectures. The first taken from the verie name it selfe, for that the worde Turke signifieth a Shepheard or one that followeth a vagrant and wilde kinde of life. By which name these Scythian Tartars haue euer beene noted, being called by the Greekes or the Scythian shepheards. <2.> His second reason because the Turkes (in his time) that dwelt in Asia the lesse, to wit, in Lydia, Caria, Phrygia and Cappadocia, spake the very same language that these Tartars did, that dwelt betwixt the riuer Tanais or Don, and the countrey of Sarmatia, which (as is knowen) are these
Tartars called Crims. At this time also the whole nation of the Turkes differ not much in their common speech from the Tartar language. <3.> Thirdly because the Turke and the Crim Tartar agree so well together, as well in religion, as in matter of Traffique neuer inuading, or iniurying one another: saue that the Turke (since Laonicus his time) hath encroached vpon some Townes vpon the Euxin Sea, that before perteined to the Crim Tartar. <4.> Fourthly, because Ortogules sonne to Oguzalpes, and father to Ottoman the first of name of the Turkish nation made his first roads out of those pans of Asia, vpon the next borderers, till hee came towardes the countreys about the hill Taurus where he ouercame the Greekes that inhabited there: and so enlarged the name and territorie of the Turkish nation, till hee came to Eubæa and Attica and other partes of Greece. This is the opinion of Laonicus, who liued among the Turkes in the time of Amurat the sixt Turkish Emperour, about the yeere 1400. when the memorie of their originall was more fresh: and therefore the likelier hee was to hit the trueth.