Are the potentials of the electrolytes equal in Galvanic cell?
There is a potential difference between an electrode and an electrolyte, but it is not directly measurable and the electrolyte potential is rather undefined.
We do not use electrolyte potentials as reference potentials, as reference must be well defined. By mechanical analogy, it would be like defining the relative height wrt a level that is not clearly given. By other words, the conventional electrode potential is NOT the potential difference between the electrode and electrolyte, even if this difference does exist.
By convention, the electrode potential is considered the potential difference between the electrode and the standard hydrogen electrode(SHE) $E - E^{\circ}_{\ce{H2/H+/Pt}}$.
The cell potential is called rather the cell voltage or the cell electrode potential difference. The cell voltage is difference of electrode potentials, which are difference of electrode potentials wrt SHE.
$$U_\mathrm{cell} = E_\mathrm{cathode} - E_\mathrm{anode} \\ = ( E_\mathrm{cathode,absolute} - E^{\circ}_{\ce{H2/H+/Pt}}) - (E_\mathrm{anode,absolute} -  E^{\circ}_{\ce{H2/H+/Pt}})\\
= E_\mathrm{cathode,absolute}  - E_\mathrm{anode,absolute}$$
Note that cathode is the more negative one for electrolytic cells, but more positive one for galvanic cells.
The cell voltage and the potential electrode difference wrt SHE ( = the cell voltage if the other electrode were replaced by SHE ) does not depend on the electrolyte potential.