The string exists (and is translated! It's a big plus), but the game code doesn't keep track of "why" the ship is going home. At the moment, it uses the same boolean (_lowFuel or something), in both cases :
- mandatory back to base because of lack of fuel
- mandatory back to base because mission completed.
Post-mission includes some tasks which are instantaneous, they don't wait for the ship to go back home :
- transport of live aliens to detention
- wounded soldiers start "healing"
- loot items (including large "tile" items like UFO power source) into home base stores
- reload spent items, drawing from base stores (clips, but also anything destroyed in explosions) Inform player of missing items.
It's a simplification which is consistent with the fact that the game doesn't expect the ship to do a second mission before coming home.
Having a ship "chain" missions would require a big rethinking, to get the behavior a player could "expect". For example, you don't expect human-made ammo to be magically transferred from base stores to the ship. But at the same time you'll want expended plasma to be replaceable with what you find on the aliens. And also, even if you do multiple missions with less and less ammo, on final return you probably expect the game to re-equip the ship back to its original loadout: What the ship contained when it left the base, not what it had when the last mission started. Even the individual soldiers equipment may need to be restored back to its "ideal" load, rather than the last mission's.