HMS Hood's armour scheme over her magazines sourced from
http://www.navweaps.com/index_inro/INRO_Hood_p1.htm:
Deck armour: 2in upper + 3in main + 2in lower... Now, separate layers often don't work as effectively as a single homogeneous layer, but that's still the equivalent of somewhere north of 5in of total protection. The French 13in gun could punch through a 3.5in deck at around 25000 yards or a 4.3in deck at a bit over 30000 yards (going with the Navweap's "USN Emperical formula" numbers, Facehard numbers for British armour gives penetrations of 2.7-2.8in and 3.5-3.7in respectively). So, plunging fire from the French vessels is effectively incapable of causing a magazine explosion at ranges where optical fire control can reliably generate hits. Damage to more lightly armoured areas of the ship cannot be ruled out.
Belt armour: Varying between 12in and 5in inclined at about 12 degrees. Not too impressive at first glance. But, going through the 12in belt into the magazine gives angles of fall between effectively zero and about 28 degrees: angles of fall less than 10 degrees (equiv. range 18000 yards using Facehard figures) would see the 2in lower armored deck bounce the shell
even if ignoring the belt and 2in sloped armour linking the main deck armour to the belt; fall angles over 16.9 degrees (equiv. range of 24000 yards) see the 12in sloped belt
alone stop the incoming shell. If we assume the combination of lower deck + sloped deck to be equivalent to about an extra 3in added to the belt then there is no range at which the ship is vulnerable.
Through the 7in belt? Angle of fall between 11 and 58 degrees can potentially put a shell in the magazine (shallower and the shell flies out the other side without hitting the magazine). The belt alone is invulnerable at fall angles greater than 39 degrees (32000 yards), 2in deck, as above, bounces shots by itself at less than 10-11 degrees (18000 yards). Using the "first guess" above of sloped deck plus lower deck approximately equal to 3in of armour, we have a possible zone of vulnerability between 27000 and 29000 yards
Through the 5in belt? Fall angles greater than 17 degrees for trajectories into the magazine. The 3in deck comes into play and,
by itself, bounces everything fired at ranges less than around 28000 yards (Facehard numbers), while the belt alone defeats everything fired from over 36000 yards. Given all plausible trajectories require passing through three or more of the 5in belt, 2in upper deck, 3in main deck, 2in sloped deck and 2in lower deck/magazine roof it seems reasonable to conclude the ship is effectively immune to magazine explosions arising from shells passing through the 5in belt.
TLDR: Plunging fire ineffective; fire through 12in belt ineffective; fire through 5in belt ineffective; fire through 7in belt likely ineffective, though potential weakness around 28000 yards.
Given the French 13in is generally superior to their 13.4in catastrophic and spectacular destruction of HMS
Hood looks horribly unlikely. Crippling shots to the engine rooms may be possible (don't have a cross-section of the armour scheme around there at hand)