The thing I can't figure out is, as the Federation added member planets why didn't Starfleet incorporate those fleets into Starfleet?
Starfleet was an earth originization correct!?
There *was* an Earth Starfleet, which presumably became part of the Federation Starfleet when the Federation came together. I'm not sure why you have concluded other member planets did not integrate their fleets as well. It seems perfectly reasonable that they would have.
The mature Federation Starfleet ships are designed and constructed by the combined efforts of all members. Earth is merely one member. The ships have English written on them because it's an American TV show, and humans appear common in Starfleet because makeup costs money and time. However, the creative intention was to show the Federation Starfleet as a combined service of, by and for all the Federation members.
There are probably between five and eight thousand ships of the line in Starfleet in TNG era. Before the Dominion War, it is probably around the higher figure, and afterwards it's probably the lower figure.