  Linear spin wave theory provides the leading term in the calculation of the
excitation spectra of long-range ordered magnetic systems as a function of
$1/\sqrt{S}$. This term is acquired using the Holstein-Primakoff approximation
of the spin operator and valid for small $\delta S$ fluctuations of the ordered
moment. We propose an algorithm that allows magnetic ground states with general
moment directions and single-Q incommensurate ordering wave vector using a
local coordinate transformation for every spin and a rotating coordinate
transformation for the incommensurability. Finally we show, how our model can
determine the spin wave spectrum of the magnetic C-site langasites with
incommensurate order.
