This again? There are no Croatian and Serbian languages, only dialects of Serbo-Croatian as we have definitively established earlier in the thread. Štokavian comprises the majority of Serbo-Croatian speakers, and it is the most conservative of all extant Slavic tongues if Matasovic is to be believed.
The only language, that has a word Serbian in its name, was abandoned in the early 19th century in favor of "Illyrian" which was
de facto Croatian standard before the official standardization. The term Serbo-Croatian was invented in 19th century as well.
What was Serb cultural contribution to the language spoken by Croats? Except few borrowings from the period of forceful unification - nothing. More borrowings came from Latin, and that is not a sufficient reason for Croats to call their own language other then Croatian. And indeed, nobody of Croats ever called their language Serbo-Croatian. It is the name used by scholars. It has a meaning for linguists, but
not for speakers.
What needs to be explained then is how Serbians came to speak Štokavian. Note in this context also the ethnonim 'Serb', which probably quite simply means 'kinsmen, one of us', derived from the Proto-Slavic word for 'suckle' - a very common naming convention among primitive tribes called 'kinship by milk'. In contrast to the very conservative nature of this ethnonym, the Croats adopted a name that in all likelihood is not even linguistically Slavic.
The picture that emerges is that the ancestors of the Serbs were probably an extremely conservative group who never changed their language or traditions much.
Most ancestors of Serb were indeed conservative, but they were, as data showed, mostly Balkan natives who weren’t Slavic speakers.
If we analyze Western South Slavic major dialects (that linguists call “Serbo-Croatian”) then we can observe the following relations:
1. Čakavian – Croats
2. Kajkavian – Croats
3. Štokavian-Ikavian – Croats
4. Štokavian-(I)jekavian – Croats, (“Western” Serbs)
5. Štokavian-Ekavian – Croats (Ekavian from Kajkavian!), Serbs (Ekavian probably from Torlakian)
6. Torlakian-(Ekavian) - Serbs
Number 4 (Štokavian-jekavian) was a language of Croatian literature of Dubrovnik since 16th century. It is not detected in Serbian literature before the reform of 19th century. It is now a Croatian standard. (It was proposed to be the Serbian standard as well but Serbs never accepted it, instead a hybrid version under number 5 was accepted. (Stokavian-Ekavian)).
It is sometimes funny to see someone call genuine Croatian dialects - Čakavian, Kajkavian and Štokavian-Ikavian – Serbo-Croatian.