Historically, liturgical usages were not mandated above the local level. (When I say historically here, I’m especially referring to the first 10 centuries of the church.) This did not mean that liturgical change and adaptation did not take place or that it only included local elements and forces. On the contrary, from the early period regional and local churches borrowed practices from other local churches, often spread by church visits or (later) pilgrimages to the great shrines. These borrowings were not centrally authorized but were adopted because they fit with the local community’s sense of “rightness” about the liturgy.
Languages usually function like that too; there are rarely centralized authorities for determining usage, and where there are such authorities, they are not particularly effective (France, I’m looking at you). There are forces for stability: mutual intelligibility, reading texts which antedate certain language changes, education, etc. There are also forces for change, obviously. This means that changes happen, but gradually. There are pockets of older usages, and pockets of evolving usages, but there is also overall and steady pressure to adapt to one another, as long as the pockets wish to communicate with one another. Here’s a quote from Mark Liberman’s discussion of this pattern for language:
The answer to your last question, “Who decides?”, is “no one”. Or better, “everyone”. Usage generally converges on a single answer in each case.
How is this worked out? Well, no one really knows, but one simple theory is presented here (with some additional background here). The basic idea is that if the members of a community start with a random distribution of distinct beliefs, and exhibit these beliefs in their behavior, and learn from (“accommodate to”) one another by adjusting their beliefs in the direction of their experience, then the community converges to a shared state. (This is true as long as “belief” is viewed as a probability distribution over categorically-distinct alternatives.)
In any case, it’s clear that language, like most other aspects of culture, is what Friedrich Hayek called a “grown” or “endogenous” or “spontaneous” order, rather than a “made” or “exogenous” or “artificial” order.
In the period often called “the era of cross-fertilization of the liturgy,” the fourth and fifth centuries when Christianity was legalized and “global” (not quite in the modern sense, but in a relevant sense), this was approximately the pattern for the development of the liturgy. Of course there were influential bishops, and their opinion had an impact — just as there are influential language practitioners throughout history (e.g. William Shakespeare) and today (LOLcats?). But their influence is felt in practice, not in decrees issued.
This self-organized ordering, the spontaneous capacity for evolving, is approximately the difference between a living and a dead language. It is parallel to “organic development” in the liturgy.
Some scholars have called the fourth and fifth centuries “the golden age” of the liturgy. I don’t agree with that assessment; I find beauties and worries throughout the history of liturgy. But the Latin liturgy’s development in that period was certainly organic; in the modern period, the Latin liturgy’s development has in general been inorganic. This is not primarily due to centralization, it seems to me (though some would disagree); rather, it is because the forces for evolution were, for several centuries, marginalized in the church (more than, say, St. Francis or St. Ambrose were in their times). Meanwhile, the forces for long-term stability (command of the ancient languages, manuscripts, and knowledge of the ancient practices) were interrupted and, when they resumed, were resumed only by a small set of scholars. The people, in whose hands liturgical reception and organic development always rest, were content to see “the tradition” as that which their parents did, rather than that which St. Augustine, St. Chrysostom, and St. John the Evangelist did. So the forces for stability only enforced short-term stability, and the forces for change were called secularists. This impeded the natural balance that allowed liturgies to develop gradually and steadily over time. Centralized legislation seems to have been a response to this imbalance, an attempt to structure liturgical development in a new way, with varied success.
–And that’s how we started doing baptism, a long wait, reconciliation, communion, a long wait, confirmation, instead of baptism, confirmation/chrismation, communion, bang-bang-bang!
But yes, I’m also implying that organic development does not proceed by fiat, and yes, that includes Trent, Sacrosanctum Concilium, and Summorum Pontificum. This doesn’t mean that fiats can’t be valuable contributions to liturgical development, but when we talk about these documents we should leave the organic debate to the foodies.