Does air density influence a football player's ability to "bend" the ball? Whilst reading an article on nasa.gov, there was a claim that I found interesting: 

At higher altitudes, the density r is lower producing a larger radius
  of curvature and a straighter path. The altitude effect helps to
  explain some of the complaints at the recent World Cup, 2010. The
  games were played at 10 different stadiums, some at sea level and some
  high in the mountains. It is much harder to bend a kick at high
  altitude.

Here r is referring to the air density.  It influences how much the ball can be curved because the sideways force on a spinning ball is the reaction force due to flow turning in the surrounding air.  
But intuitively, I feel like the difference in altitude between stadiums could probably only account for a couple of centimetres at most.  
So, assuming an identical penalty kick, how big would be the distance between targets at the highest and lowest world cup stadiums?
 A: Altitude can indeed have such an effect. As your linked article explains, one can get a rough sense of the aerodynamic force on a spherical ball by neglecting viscosity (i.e., model air as a bunch of ballistic particles that do not drag on one another), in which case the formula is1
$$ F = \frac{16\pi^2}{3} C_l \rho \omega v r^3. $$
The important point is that the force is proportional to the air density $\rho$. This makes sense: The aerodynamic force should be $0$ in the limit $\rho = 0$, and the fact that we are neglecting particle-particle interactions means that if we cut the amount of air particles in half, there should be half as many interactions with the ball and therefore half as big an effect.
The question is, how much does density change with elevation? In particular, the 2010 World Cup venues ranged in elevation from sea level to $1750\ \mathrm{m}$, so how much can density change over that range?
Air density is rarely tabulated in weather reports, but it can be determined based on temperature, pressure, and composition (mostly varying based on humidity) using the ideal gas law. Without bothering to look up these values on game days, we can get a sense of what the average difference is between a "typical" place at sea level and another at elevation $h$ using the international standard atmosphere model. In this model we have the following values for temperature, pressure, and density:
\begin{array}{l|c|c}
\text{Elevation ($\mathrm{m}$)} & 0\ \mathrm{m} & 1750\ \mathrm{m} \\ \hline
\text{Temperature ($\mathrm{K}$)} & 288.15 & 277 \\
\text{Temperature ($\mathrm{^\circ{}C}$)} & 15 & 4 \\
\text{Pressure ($\mathrm{kPa}$)} & 101.325 & 82.0 \\
\text{Density ($\mathrm{kg/m^3}$)} & 1.225 & 1.03 \\
\end{array}
In this case, the density at the higher elevation is only $84\%$ the density at sea level. Thus the aerodynamic force will be only $84\%$ as strong, and the radius of curvature of the path, $R = mv^2/F$, will be $19\%$ larger. If I apply this to a kick that goes a distance $11\ \mathrm{m}$ in its original direction and also deflects $2\ \mathrm{m}$ sideways2, we originally have $R = 31\ \mathrm{m}$. Going from sea level to $1750\ \mathrm{m}$ elevation changes $R$ to $37\ \mathrm{m}$, which results in a deflection of only $1.4\ \mathrm{m}$.
The result is a measurable change from $2\ \mathrm{m}$ to $1.4\ \mathrm{m}$, though maybe this is not too large compared to player-to-player or kick-to-kick variation. For example, this analysis predicts the deflection will be affected by changes in the spin imparted to the ball in exactly the same way as changes to air density. And of course, weather conditions can vary a lot compared to the standard atmosphere model, and this analysis doesn't take into account how people react at different altitudes.

1That article uses terrible letters for its variables, so I've changed them according to $\mathrm{Cl} \to C_l$, $r \to \rho$, $s \to \omega$, $V \to v$ and $b \to r$, bringing them more in line with standard notation.
2I'm just making these numbers up. Someone with more knowledge might correct me on more appropriate numbers for a penalty kick.
A: I believe that yes, it could. However you must also take into consideration that air density may not be the only apprehension that a player is dealing with throughout a game. As to answer your other question multiple world stadiums are covered at a sea level however the highest was located at Estadio Da Baixada, Curitiba which was  920m (3,018 ft). Depending on the situation that your depicting i'd recommend  using terminal velocity in the presence of buoyancy force
