The music sources is Album national russe (Köhler, Louis), and the particular passage is...
...before saying the music is in
G major and there isn't any
C# to make lydian, hear me out. The basic voice leading for
I6 IV would have voice leading of
^5 ^6, and if the line to the
^6 was a descent it would use a lowered seventh, like these...
If the lowered seventh version were tonicized to
C the line would become
^5 ^4 ^3. So, the point I'm getting at is the
^7 is raised rather than the typical lowered. It's even emphasized as an accented passing tone. The
F# is played above a bass/root
C for an augmented fourth.
I would think from a jazz perspective that
C chord would be labelled not as plain
Csus4 but something like
C(#4). Of course the style of this music isn't jazz, but it seems to me that augmented fourth (keep in mind it's not resolving like a dominant tritone to
C is the chord root) is the source of the lydian sound. My understanding is common practice avoided that sound by lowering the
^7. The fact that it isn't lowered, and is accented, makes me think the passage could be described as having a lydian sound, lydian tonality. Even if it is for only a brief passage.