What do you mean by language systems? I assume you mean sub-Saharan Africa, especially. I'll try to divine it and maybe this will be helpful.

What's important to note is that colonialism--and the missionaries who sometimes preceded or followed it, in non-Islamic areas--brought two relevant things rooted in the European enumerative and controlling drive that attended the modern state. The first was the need to find 'boxes' for the societies whose lands they claimed possession over. The second was the need to assure that colonial dictates were reliably spread in those same networks of communication. Both pushed for the collapsing of a vast variety of dialects--tens of thousands, literally--into a far smaller number. Whereas one had many localized variants of (say) isiXhosa in South Africa, the colonial government and the early mission societies began to divine a standard grammar and set it to Latinate characters. This privileged the variant spoken in certain areas, and by certain people who had the best relationships with the colonizer. Anyone who's tried to master isiXhosa will tell you that what's spoken in Bizana is so different from what's spoken around Cape Town that native speakers have enormous difficulty. However, the written language is the same, officially. Anyone educated at mission schools would be steeped in that written language, regardless of any dialect code-switching they might do when actually speaking with people. The same was true all over the continent, and it's worth pointing out that a lot of these cases began with African converts translating the bible (e.g., Samuel Ajayi Crowther, the truly remarkable Yorúbà-born Anglican bishop of West Africa in the 19th century).

As a variant of this process, African languages in Islamic areas were transliterated into Roman characters and made more 'standard'--Mande and Kiswahili are two key examples, and both languages have very few speakers who can read the Arabic script versions anymore. Kiswahili was a wide range of dialects that grew out of fusions between port and shore communities, but the standardized Kiswahili you'll tend to learn today outside of East Africa will be that of Zanzibar, even though people in Kenya, Tanzania, Uganda, and the eastern Congo/Rwanda/Burundi will speak something quite different, and there are Swahili creoles like Sheng about as well.

Furthering that tendency to standardize regional dialects into languages depending on missionary and colonizer identity and activity, having a single standardized written language (and one that could be spoken) also allowed for the wider growth of intellectual exchange, including anticolonial speech. Admittedly, these intellectual discussions were often the province of the colonizer's language (English and French most notably, but some in Portuguese and a little in Afrikaans), but sometimes the ability to write and read in a language very few of the colonizers actually knew was important. So during the colonial era, newspapers spring up that mix the colonial language with items that are in African languages as codified first at the missions. South Africa had many, but they popped up in the late 19th and early 20th century all over the continent. The importance of that, and the culture of literacy it helped to spread, shouldn't be overlooked--even people who could not read the papers could get that knowledge from others in the same community who could (often children). The shift after WWI and especially in WWII from anticolonial ideas among elites to a larger mass movement required these shared languages--local and colonial--that knit together larger groups of people.

In effect, colonialism and its associated mechanisms standardized languages across larger groups of people and often added the written component, at the same time that educational changes (often but not always tied to missions) assured that people would have greater access to them. The last bit of data comes from mass urbanization in the early 20th century that is still going on, which tended to uproot people and throw them together in new surroundings in ways that encouraged a greater reliance on the shared dialect instead of the community variants.

I don't know if this answers your questions. I know Kevin Shillington goes into some detail with this in his survey (History of Africa 3rd ed) but it being Sunday my more detailed works on missions, education, colonization, and language are not with me. If there's a specific point or area (or mechanism) of interest, I'm happy to dig into my data. I've had to generalize here pretty wildly, so there are always some exceptions, but the growth of broader 'ethnic,' 'national,' and even 'pan-African' identity types is tied to language broadening really closely even if very few at the time made the explicit link. Marcus Garvey's UNIA, for example, had a great reception in much of Anglophone Africa for that reason, although it had to do with English and not vernaculars.

ninja edit: I should note also that the growth of radio and the distribution of film, which are only now getting more attention among scholars, also played a role--although their reach often demanded the use of the colonizer's language, not that of African societies themselves.

not-so-ninja edit: I am of course not noting here how much borrowing took place between codified languages--and that's a significant element--but it doesn't change the point of standardization.