I am afraid that it is easy to come with translations, but it is hard to fit them in a song.
Especialy when only minor differences in meaning are involved.
Anyway, as long as the desired meaning is "for the sake of X", fpi
is correct - with only one notice: it is an adposition, thus some noun or normalized clause is required:
"Leru krr fpi fwa kaltxì si.
" or "Krr za’u fpi fwa kaltxì si
, I see "lu krr fpi awnga
kaltxì si" - "There is a time for us
", and the "kaltxì si" remains alone.
Personally, I wouldn't use fpi
, but fte
, because it seems to me much closer to the original meaning:
"Krr za’u fte kaltxì sivi.
" (still maintaining 8 syllables as your original)
My preference of fte
is also based on the fact, that fpi
is to be used with a noun, while "kaltxì si" is an action.
Completely different approach is to use the adverb nìkmar
"Nìkmar awnga fìtsap kaltxì säpivi
" - It is the right time for us to greet each other".
- here the inclusive plural pronoun awnga
directs me to use the fìtsap
construction - we are saying hello to each other, no to someone else, and subjunctive <iv>
express the possibility of action to happen.
The worst thing in translating to Na’vi is not the unability to say something, but the choice from many possible alternatives