Long ago, we looked upon a foreboding sky.
かつて人々はその空を見上げた
"Before/previously people looked up at the/that sky." (nothing about foreboding in the subtitles)
The memory of the star that threatened all burns eternal in our hearts.
全てを飲み込む明星はいつまでも心に刻まれる事となった
"The bright star that [would/does] engulf everything will always be remembered." (no substantial difference)
In its wake came an age of silence.
そして我々は沈黙の時代を生きた
"And [following that] we lived [in/through] an age of silence."
Yet with each fond remembrance, we knew: those encountered were not forgotten.
思い出を振り返る度に彼らを忘れていない事を思い知る
"Each time we look back on [our/those] memories we realise that we have not forgotten them [those people]."
That someday, we would see them again.
いつかまた会えるのではないか?
"[We wonder if we] might meet/see them again someday." (this doesn't have the same sense of 'knowing' that comes across from the English dialogue and sounds more uncertain)
Perhaps it was no more than wishful thinking.
そんな予感もただの願望だったのかもしれない
"That [such a] feeling/premonition, too, perhaps was merely a wish/desire."
But after the long calm, there are now beginnings of a stir.
長き静寂を過ごし、やがてざわめきを感じ始める
"[We] live/passed time [through] a long quiet/silence, and eventually [we] start to sense a stirring."
The reunion at hand may bring joy, it may bring fear.
その再会に喜びと恐れが入り交じったとしても、
"Even if that reunion is mixed with joy and fear," (continues to the next line)
But let us embrace whatever it brings.
今は抑えきれない感情を歓喜としよう
"[for] now let us [say/take these] irrepressible emotions [are/as] joy" (more positive 'let's be happy about this' message)
For they are coming back.
彼らが帰って来る
"They are coming back"
At last, the promise has been made.
その時は遂に約束された
"That time [their return] has finally been promised."