It's difficult to tell (since each word contains at least one problem) but it looks like the order of the lines in the "Elvish" translation and the English are different.
For example the first English line matches the fourth "Elvish" line:
Let's walk, Brothers, on the warpath
Pado nin teryn, na i maetha ne irath
But it rather says:
Walk me brothers(!), to the to-fight(!) in(?) thepath [sic]
If I were to address all of the problems, I would end up writing a novel, so I will spare myself and everyone else here. What I will say is that, if the "Elvish Community" of those French LARPers care even a little bit about authenticity, then they should stop repeating this thing immediately. I will give proper translations if it would help. The English translation does not quite match what I can gather from the "Elvish" one, but I will stick to the English one in any case. Now...
Quenya:
Á lavë pata, hánor, an' ohtatië.
"Let [us] walk, brothers, on(to) (the) war-path."
Hiruvalvë alcar úvoronwion sercessë.
"We will find glory in (the) blood of (the) unfaithful."
Ai, Oiror, liltarion (†)talpati nin hehtauvaldë.
"O, Eternal(one)s, footprints of dancers you will leave for me."
Tula, lava yulë lan ohtan horalvë.
"Come, let [us] drink while we wait for war."
Cottonna, Oiror!
"To (the) enemy, Eternal(one)s!"
[Tried rhymes, except for the last line.]
Sindarin:
Davo baded, hoen, an i-nagrad.
"Let [us] walk, brothers, to the battle-path."
Hirathab aglar mi-sereg in-alvoryn.
"We will find glory in the blood of the unfaithful."
A, Uiryn, (†)delphaid lilthyr (‡)eglathadh annin.
"O, Eternal(one)s, (†)footprints of dancers you will (‡)leave for me."
Tolo, davo ýled (#)lo dharthab an i-nagor.
"Come, let [us] drink (#)while we wait for the battle."
An i-chyth, Uiryn!
"To the enemy, Eternal(one)s!"