Why Dunkerque needed range beyond the Mediterranean

Did Dunkerque's long range reflect a possible need to operate in Indochina?

Yes. France planned for both Mediterranean service and a Far Eastern war fought alongside British naval power.

France could not optimise Dunkerque for one enclosed theatre because the ship also needed the range to reach Indochina. French planning assumed that a British battle fleet would be involved in such a campaign, making allied logistical compatibility valuable. The later French preference for 15-inch guns fit that expectation: British and French shells were not interchangeable, but their propellant requirements were similar enough to offer some practical scope for adaptation. Range and calibre choices followed imperial geography and coalition warfare.