So, the Concentrate trait has no mechanics, but other mechanics might refer to it?
Yes. The most important ones are that casting a spell with verbal components have the Concentrate trait by default, and you can't use Concentrate actions/activities while raging.
How easy is it to play different settings in P2?
How baked in is the Golarion flavor into the classes and rules, in practice.
For example,
The stereotype of Nordic peoples as the "fur-clad raiders" Ulfen is uninformed and a bit offensive.
Also, I am less interested in the gods scene, and the core Cleric class seems to require it.
What do P2 players do for non-Golarion settings?
The biggest thing that's part of the rules themselves are deities and their connection to Champions, Clerics, and various Divine spells. Mechanically, deities have the following relevant parts:
- Alignment (which affects some spells – e.g. divine lance is a cantrip that deals damage of a type corresponding to one of your deity's alignments, so a follower of the LG god Torag can deal lawful or good damage with it).
- Follower's alignments (restricts which alignments can get divine oomph from the god in question)
- Divine ability (as in ability score, mainly used in one or maybe a small number of backgrounds)
- Divine font (whether clerics of the deity cast harm or heal with their Divine Font ability, and various effects that follow from that; in almost all cases this is linked to alignment)
- Divine skill (a skill all champions and clerics of the deity are trained in)
- Favored weapon (a weapon all champions and clerics of the deity are trained in)
- Domains (domains that have focus spells that champions and clerics can learn via feats)
- Alternate domains (more domains that require a particular feat to take, often considered part of alternate interpretations/traditions of worshiping that deity)
- Spells (a set of usually three spells that are considered divine spells for clerics of the deity – some deities give more spells, but that's pretty rare)
There are also various spells which require that you have a deity, usually because they have an effect depending on which deity you worship (e.g.
righteous might which turns you into a battleform with weapons determined by your deity).
The canonical (ironically) solution for people who want to cast divine spells without worshiping a deity is to be an Oracle instead.
As for ethnicities and rules connected to those: there are some ancestry feats, mostly in books in the Lost Omens product line (the Golarion setting). I don't think there are any in the core books.