Languages and a question

First, about languages, weapons etc IMC: on my homeworld everyone speaks
their native tonge and Common, which is the holy language thought at
school/church. A poor peasant may not speak it very well, but enough to
make himself understood.
In other spheres, languages may translate upon entering the sphere, they
may not, or it may vary for each individual, all randomly determined (or
rather, determined by me on how much I think it would improve the game.
Also, most SJ-ports speak a common Trade language, an artificial
language created by the Arcane (*not* their own native language though)
specifically for the purpose of easing trade.
For the rest, I agree with Charles Remis' post about languages, and I
think language problems can create tremendous fun, but it is something
to be used sparingly. My players know that when they land in a strange
port they can reasonably expect most people to speak Trade, and that
this will change gradually should they choose to travel inland.
As for racial prejudices, they  are *not* the same everywhere; Riann
doesn't have them and it varies greatly on other worlds, possibly
including elves,  and dwarves, with orcs against everyone, but is just
as likely to be between humans and gnomes, with tabaxi as the general
baddies.
With weapons, I occasinally try to throw in exotic stuff like that
what's-it-called that the Klingons use, but usually the difference is
only cosmetic, both because my imagination runs short here and because I
strtongly suspect that most everyday items have their shape because its
functional that way, and so is going to look much the same everywhere.
Some worlds may favour oil lamps over candles, but oil lamps everywhere
involve a container, a burnable liqiud, and a wick.
For weapons it can be argued things are slightly more difficult (a
katana is not a claymore), but I'll deal with that when I think it will
improve the game enough to be worth the trouble.

Now, all this talk about language has made me wonder on something
different entirely; namely experience points.
I was thinking about languages and slots, and how you get only so many
slots at the various levels, when I suddenly wondered why exactly you
need an increasing amount of xp to attain a new level.
After all, you do not get an increasing number of slot, nor can you do
more with a "high level slot" than with the ones you get at L1.
Why would it be so much more difficult to learn a new language at L20 than it is at L1?
For the spellcasters this can be explained by their getting access to
higher level, and presumably more difficult, spells, but this is not so
for the fighter or the thief; one weapon is as difficult as another (1
slot), and the thieving skills don't change much either.
So, can anyone give me a good explanation for the increasing amounts of
xp, other than the wish to make highlevel characters rare?

Hoping for some replies this time,
Stardancer