Hola Ruediger

It's not a literal translation in this case because it wouldn't work in English if we used the same structure using the gerund "being its official number of inhabitants xxx people".

In Spanish there is flexibility for this type of sentence so you can swap the elements of the sentence, however it sounds more natural to use "siendo" in the beginning. But you can either say:

siendo xxx personas su cifra oficial  

siendo su cifra oficial xxx personas

Both options are fine.

If we think of a simpler example sentence you may see it more clearly. Here for example you can also swap the different elements:

siendo su perro negro...

siendo negro su perro...

being his dog black... (his dog is black)

Or with a different verb also:

teniendo María muchos amigos...

teniendo muchos amigos María...

María having lots of friends... (Maria has lots of friends)

I hope it clarified it

Saludos

Inma