Proper kitchen temperature for doing "fermentation"

The proper temperature is the average temperature range of the type of room in which the fermentation was historically done by the culture which invented the specific fermentation dish.

I know this is not what a modern cook wants to hear, but there is no easier way to put it. Recipes were invented to work at a certain temperature, and because this was such a ubiquitous part of the whole thing, and historically so hard to change, that it does not appear in the recipe itself.

For example, if you are creating a sourdough starter, you can easily stay around 20 Celsius. The range is rather wide, but the exact smell of the starter will depend on the temperature you had, as you're likely to get slightly different bacteria at 25 Celsius than at 18 Celsius.

Other recipes will need another temperature. You seem to need well above 30 Celsius for dosa batter to ferment properly. Other cases, such as sauerkraut or wine, need a temperature more common in old European cellars, between 10 and 15 Celsius. Salami might fare best if you have a bit less than that. Yogurt needs to be quite warm, but depending on the temperature you choose within the "warm" range, you will get sharp vinegary bacteria or milder lactic bacteria. And so on.

If you have a modernized recipe which directs you to use room temperature, stay in the 20-25 Celsius range. But if you are not sure the recipe has been made with climatized rooms in mind, you might need to do some research.

Trial by error. Sandor Katz in "The Art of Fermentation" indicates tropical "room" temperatures may run a fermentation to completion in just half a day, while more pole-trending latitudes could instead require multiple days. Also of concern may be mold, especially for a five day ferment without rotation and feeding of the input, though that will vary by location, what is being fermented, whether the container used is somewhat open or a rubber gasket jar, etc. The desired duration will also depend on how sour one wants the resulting product, etc.

Temperature while important by no means covers the whole story--what container is best used?--what adjustments must one make if that container is not available, or is unsuitable?--how to deal with mold?--what happens when the inputs have regional differences unaccounted for by the recipe?--which inputs are best to use?--how sour should the resulting dish be?

Trial by error has been an essential tactic for maintaining a sourdough colony (modern flour is terrible, need whole grains), Moroccan preserved lemons (boy howdy mold problems but think I have a good fix for that), beet kvaas (duration is critial, as left too long hey there's that Seattle mold), kraut, mead, etc.