You're very welcome. The piece is in Gm and in bar 10 the F# is a dead giveaway that the chord in question is a D7 (D dominant 7). In the intro, it's certainly a D 7th chord, but ambiguous, because there's no 3rd. One possibility would be to play the D - A - A (the bass notes, not the chords) on counts 1, 2 and 3 or D, A, and C or D, C and A. The intro alternates between Gm and this ambiguous D seventh chord, leaving it up in the air what key it's in.

The harmonies of this song are perfectly straightforward but of course if one hasn't something before, one has to find one's way. This arrangement is suitable for a typical oom-pah-pah waltz accompaniment for accordeon: that's basically what's written.

Starting at bar 101, it briefly looks like it wants to modulate to E-flat. The chord A-flat major is the subdominant of E-flat, followed by the B-flat dominant seventh chord in bar 102, so if the next chord was an E-flat major or minor, you would have a cadence. However, the song changes its mind at the last minute, plays an A-flat major seventh chord (which includes the E-flat as a sort of consolation prize) and then just goes back to Gm for the ending, which is a little unusual.