What else do I want to say about the difficulty of translating this (any) Baudelaire poem?  Let's look at the sestet, first in French, then as I've provisionally translated it:

Un éclair...puis la nuit! --Fugitive beauté

Dont le regard m'a fait soudainement renaître,

Ne te verrai-je plus que dans l'éternité?

Ailleurs, bien loin d'ici!  trop tard!  jamais peut-être!

Car j'ignore où tu fuis, tu ne sais où je vais,

Ô toi que j'eusse aimée, ô toi qui le savais!

Rhyme scheme (lost in translation, but note that Lowell kept rhymes in his Baudelaire "imitations," which don't include this poem, opting for different losses): aba baa.   Lines of 11-12 syllables.  So what if one  keeps a 10-syllable line, as a constraint, respecting the contents of each line and its break?  Generally, English poetry has evolved a slightly shorter line than Italian or French hexameter, possibly because those languages  1) are inflected 2)are less rich in monosyllables than English 3)have their tonic accents on the penultimate or anti-penultimate syllable.  Here's what I've got so far:

One flash of lightning…then night! —Oh beauty

Whose glance jolts me to life: shall I not see 

You again, except in eternity?

Too far away! Too long! Never maybe!

I don’t know where you're going.  Nor you me--

You whom I’d have loved—and you knew, Lady!

Losses?  

Stanza 1:  first line a whole word, "fugitive"; I expanded "éclair," to "flash of lightning," because "flash" alone, in English isn't, I think clear, though I could have second thoughts on this, which would free up some syllables for "fugitive," except that "fugitive beauty" sounds old hat, and the rhythm doesn't work.

Second line?  I fussed with "shall I not see," very formal, maybe too formal; also not sure about the line break, though I was sort of tickled my largely unintentional aaa aaa rhyme scheme (?) 

Third line?  Eternity is a big, abstract word for English to handle, but are there any alternatives?

Stanza 2:  "flee" again.  Nobody "flees" in English, not lately.  Baudelaire packs four exclamation marks and several internal rhymes into these three lines  Not to mention the surprise last half-sentence, which captures her collusion in his feelings:  the "I know she knows I know".  Is my last stanza taut and exciting enough?

I didn't mention that the second quatrain, quoted a few posts earlier, describes her legs, suggesting, without saying, she's flicking her dress around enough to bare it.  Again all the delightfully perverse ambiguity of the encounter. In "La Fanfarlo," a Spleen story, Baudelaire has a detailed description of the actress in her loge lacing up her boots:  "Cette jambe était...l'objet d'un éternel désir.  Longue, fine, forte, grasse et nerveuse... ."  But her leg in the poem is described as "noble," another of those outdated, abstract words.