I've discovered that dungeon block N0000052 is the best block for a dungeon to start in, since you start with the entrance facing a door that leads to a "prison" of 7 little rooms which all have the same monster in them. Dungeon inhabitants are deterministic based on the player's level, so at certain levels you can be sure to find certain monsters in a huge group here for practicing language skills.
For example, if you go to The Conclave of Gherk-i in Mournoth at level 6, there will always be 7 Centaurs right in front of you to converse with (frantically, before fleeing).
I've been adding this info here, and I've almost found perfect leveling spots for all monster languages. At the moment still need to find Dragonish and Nymph, and maybe some better spots for Daedric.
But it occurred to me that rather than searching manually using Daggerfall Modeling, it might be worth just asking whether a tool or method exists to reverse-lookup dungeons based on their starting block. Would it be possible to generate a list of all dungeons in the game that start at N0000052?
Another decent starting block is N0000013, which has a "grotto" with a stone bridge immediately to your right after entering, with 4 of the same enemy in the top corners of the room. Would be nice to know dungeons that start in N0000013 as well, but there's really no comparison to N0000052.
In fact, N0000052 is good for all kinds of training, having that many enemies and that much treasure right in front of you on every entry, so it'd be nice to have a list just for general use for everyone!