For computers, I would advise 3 prong unless necessitated by design. If using 2 prong, it needs to be double insulated. Usually anything over 200W requires a ground but I'm not sure where that comes from. Might be an international standard somewhere.
Not sure what you mean by "galvanic," as a quick dictionary look comes back with: "relating to or involving electric currents produced by chemical action." Optocoupler has little to do with this situation, as optocoupler is used to electrically separate signals.
The technical answer is that power bricks are "double insulated" in American terms (not sure what they call that in EU). As you mentioned, it is insulated by the plastic enclosure, and the second part, to be exact, is electrically/physically separated. The output side is not connected electrically to mains (wall power) via the use of a *transformer*. Pretty much all modern AC/DC power supplies use a transformer to isolate from mains power.
And finally, a number of factors necessitates the use of a grounding prong. If it is commonly used near water, a grounding prong and GFCI is recommended. If it used nearby vulnerable groups, children or hospital patients it must be grounded or double insulated. I am sure there are many more factors that are outside my experience.
