Most of the Norteña, music that has been popular for many years is based on the major scale.  We rarely use any notes from outside the scale except as accidentals (passing notes).
But if you want to play some of the more advanced styles, especially the Tejano music played by artists like Flaco Jimenez or Albert Zamora, you will need a bit of understanding of the scales they use.
Lets look at the Key or G.
GABCDEF#G
Rock and Blues rely heavily on the dominant seventh chord.
Instead of playing a G, it is very common to use a G7 as the first chord in a song.
What are the notes of a G7 chord?
G B D F
By lowering the seventh note in the scale and adding it to the G major chord you get a G7.
But that changes the scale.  If you lower the F# in the G major scale, you now have the same scale as a C major scale, but starting on a G note.
Conclusion.
Use the scale from the 4^th interval of the Key when you play a rock or blues style song.
In G, use C
In C use F
In F use Bb
In D use G
In Bb use Eb (Good luck on that one)
This is interesting in that Harmonica players do exactly the same thing.  Blues harmonica players use a C harmonica when they play blues in G.  Unfortunately, they can bend notes but we cant.  Why is this important to us?  In case you havent noticed, the Diatonic Button accordion is really just 3 harmonicas in a box, with buttons and a bellows.