That entirely depends on the setting your DM is using or has created.
Apart from setting-specific languages, it is typical (though not guaranteed as it's again up to your DM) for there to be one language for each major race; i.e. elvish for elves, goblin for goblins, dwarvish for dwarves, etc., etc. This is typical of AD&D simply because such "racial languages" are mentioned in the write-ups for the playable races in the Players' Handbook and the entries for creatures in the Monstrous Manual.
When considering what languages to choose, always consult with your DM.
There are major differences, but nothing too insurmountable.
- AD&D 1E adds non-weapon proficiencies in DSG, WSG, and OA, while they are core in AD&D 2E
- 2E allows points to be spread amongst thief skills, rather than using a fixed table per skill.
- 2E groups classes differently, and lacks the assassin and monk.
- Minor differences in several tables
- several spell changes.
- minor NWP changes
- Psionics are incompatible between editions - pick one, and only one, and use it exclusively.
- 1E Class List: Fighter, Magic User, Cleric, Illusionist, Thief, Druid, Paladin, Ranger, Monk, Assassin, Fighter/Thief/Bard; adding UA adds thief-acrobat, Cavalier and Barbarian. Dragon adds a dozen or more NPC classes of various quality levels. DLA adds 4 classes of knights ad 3 of wizards, plus tinkers. OA adds several oriental classes as well
2E class list Fighter, Magic User, Cleric, Thief, Bard, Paladin, Druid, Ranger, PsiHB adds Psionicist. Sages and Specialists adds a dozen NPC classes, all pretty consistent power levels, and many playable as adventurers. Dark Sun adds a few more classes, including Gladiator, and in Dragon Kings, adds more thief abilities; the Thief abilities are in some later 2E Player's Option series books.
Most 2E DM's used some 1E materials. THe key is picking your core edition, and what you'll cross over. Monks and Assassins crossed over VERY easily. DSG/WSG non-character rules can be used as is.
Again, Pick your reference edition: the rules are subtly different in many places, and it trips people up. So use one as your ruleset, but allow the stuff you want from the others as a port-in.
Also, note that monsters from other 3E/3.5E/PF can be used easily enough simply by looking up THAC0 from HD, finding the AD&D AC by taking 20–(d20 AC), and spells directly across by name (even when they do very different things).
Note also, porting in monsters from 5E is equally as doable, with the same basic caveats as from d20. Advantage/Disadvantage is also easily imported. Magic weapons and armor are far less portable, because the scale of AD&D magic items is +1 to +5, with a few very rare +0's; 5E is +0 to +3, and +0 with a bonus of some kind is extremely common (eg: Flametongue weapons are +0 to-hit, but do +2d6 fire damage).
Best Answer
Go wander outside your house or apartment or whatever and look for a wooded area. Write down a description of it. You've created an environment, then when you read that to your players, they are in it.
Does it have corridors or rooms? No. It might have paths, and clearings that might make good places to have something happen, but the outdoors largely lack the constraints of a dungeon environment.
Do you need a large scale map? If you do, You could map a wilderness area and place some encounters. The first edition DMG has a method for randomly generating type of terrain in each map hex. Consider reading it, or running one of the old 1e adventures to get an idea of how the game works in the wilderness. Or use a real wilderness map from a local national park (or from Google).
Do you need a tactical map? Because often you don't, you know, the description and your players' imagination is more than sufficient. You can make maps of such areas easily, here, check out this random wilderness map generator.
(Note that this question is tagged 2e, so the 4e/late 3.5e concerns about having tactical maps for everything are not very relevant - in 2e we used minis and tac-maps pretty rarely.)