I wanted to analyze these 2 poems that are in the volumes with Renji's covers. I've read some English translations, the Viz ones and the fan translations, and they are a bit different from the translations that these poems have in the official translation of the volumes published in my country, Spain. And I think those small differences, in my opinion, make that the poems have more sense and helped me to understand them better.
届かぬ牙に 火を灯す
あの星を見ずに済むように
この吭を裂いて しまわぬように
The English fan translation of this poem was this:
I will light a fire on an unreachable fang,
so that I won't have to see that star,
so that I won't let it tear at my throat.
Light a fire to the fang that cannot be reached
so that I do not have to see that star
so that I do not slit this throat
These 2 translations are basically identical.
The Spanish (from Spain) translation (I don't know if I got to translated it well to English, but I hope the difference is understood):
In his fangs, which cannot reach it, a flame is lit,
so that he does not have to look at that star,
so that his voice does not fade.
(In the original Spanish:
En sus colmillos, que no pueden alcanzarla, se enciende una llama, para no tener que mirar a esa estrella,
para que su voz no se apague.)
The last two verses don't really change , they basically mean the same in all the translations. Well, in the Spanish translation the poem is written in the third person, but, whether this is right or wrong, that doesn't change the meaning. The difference is in the first verse. In the first two translations they both talk about a fang that cannot be reached, but in the last one it's the fangs which can reach ("it", supposedly referred to the star). And I think this makes more sense knowing Renji's story in that volume, and the title of the volume "A star and a stray dog", and what Renji says in chapter 98: "I bark at the star but I don't have the courage to jump and grab it", the dog is Renji, and so the poem talks about fangs, his fangs. He can't reach the star, who is supposed to be Rukia, and she appears with an ethereal appearance on the chapter cover where their story is told and that it basically has the same title as the volume cover:
"An unreachable fang" of the other 2 translations really doesn't make any sense if Renji himself is that fang/s (can't he reach himself?). The next parts of the poem are similar in all the translations, and the meaning looks more understandable,
"so that I do not have to see that star
so that I do not slit this throat"
"so that he does not have to look at that star,
so that his voice does not fade."
Here Renji is saying that his suffering is so much that he would bark so much (continuing with his metaphor with a dog) to the point his throat would get slitted, and thus his voice would get lost, all of this because he can't reach the star, Rukia. So, to endure this suffering, it would be better for him if he doesn't see the star, he yearns for her so much but he can't be with her, or, other interpretation might be that he can only look at her, from afar, and he wants more than that, so it would be "so I do not have to (just) see that star". So that seems to point at the 40 years where Renji and Rukia didn't talk to each other. Renji wanted to go to meet her, but he is told is better for both of them if he doesn't, so he tries with all of his willpower to avoid her, the same as she did, because at least this way the pain is less.
Then we have that "light a fire" "lit a flame", which appears in all the translations. Why a fire? What does this mean? I had some ideas but I wasn't totally sure, until I saw the volume 73 poem and the drawing above it that Kubo drew:
It's Zabimaru shikai, Zabimaru with the appearance of having been burned. Plus, Zabimaru shikai is thooted, it has what looks like fangs. So in the volume 11 poem, the fangs have to be a referrence to Zabimaru, which is a symbol of Renji himself too. He lighted a fire on Zabimaru in the volume 11 poem. Now, to continue the explanation about why a fire, the volume 73 poem gives the answer. There are differences in the different translations too:
牙より滴る火は消えず
刃立つ野を焼き払い
友の姿を炙り出す
The English fan translation:
Fire, dripping from my fangs, does not disappear
But keeps the battlefield burning
Bringing the true face of a friend to light
The fire that drips from fangs does not fade
The blade burns away the field
Revealing my hidden friend
The Spanish (from Spain) translation:
The fire that falls from the fangs does not go out.
rising up in arms and razing the fields with fire
I reveal the form of my friend.
El fuego que cae desde los colmillos no se apaga.
levantándome en armas y arrasando a fuego los campos
revelo la forma de mi amigo.)
The first 2 verses are similar in all the translations and they basically mean the same, and with this poem is when I realized that fire is referred to Renji's power, his zanpakuto power, this was more obvious seeing what his new bankai did in his fight with Mask, Renji burned him. In both poems Kubo uses "fire" to describe Renji's power, it falls from his fangs, his zanpakuto. In volume 11 the fire was lit, his power started being really developed in a serious way, Renji needed to get stronger because he couldn't reach the star, Rukia. Renji knew that he had to get stronger to be able to see and talk to Rukia, their relationship depended on his power, that was what motivated him to get more, and thanks to his power he became a vice-captain, what put him on par with Rukia's social level. Whether or not he thought he was capable of achieving this goal, he at least felt he was doing something to be able to be with her, reach the star.
However, in volume 73 poem, it does the impression that he is not desperate anymore, surely because he fixed his relationship with Rukia, but his fangs continue still dripping fire despite that fact. He is still getting stronger and developing his power, but his motivation to get stronger has changed. Now this power that continues falling from his zanpakuto fangs has a different goal, now his power simply helps him to fight: "but keep the battlefield burning", this means that he is still developing his power, but to be a better fighter, it's not anymore a way to reach the star that he couldn't reach, because he has already reached the star, he already has a relationship with Rukia.
About the last verse, here is where there are differences in the translations that can change the meaning, depending on how people interpret it, what "friend" Renji is talking about. In the fan translation it looks like is referred to a person, the Viz one is more ambiguous, and the Spanish translation is more clearly referring to a "thing", not a person. When I read "I reveal the form of my friend" was clear to me that he is talking about the new form of Zabimaru bankai, he reveals the form of his new bankai, his friend is Zabimaru, this basically means that Renji is now on good terms with Zabimaru and because of that his zanpakuto spirit revealed him its true power. Renji's cover and his poem are on volume 73, but I think Kubo wrote this poem thinking about the chapters where Renji reveals the new form of his bankai. At that moment Renji could't have that volume cover (the volume 63), but when he finally put Renji on a cover he wrote a poem dedicated to that moment, the reveal of his new bankai.