J'aimerais quand même te direRoughly, this means "Nevertheless, I would like to tell you / All that I could write / I drew it from the ink of your eyes." Ugh; it just sounds so contrived in English. The general idea is that the woman's eyes are a source of inspiration to him, but the French words describe that inspiration with the metaphor of a quill pen drawing ink with which to write. The woman's eyes, then, are not "inky", but are rather an ink-well that has provided for all he could write: a bottomless well of inspiration.
Tout ce que j'ai pu écrire
Je l'ai puisé à l'encre de tes yeux.
But when you translate into English and opt for the "inspiration" concept over the literal French words, the person hearing it in English is missing the beautiful poetic metaphor: "Nevertheless, I would like to tell you / All that I could write / It was your eyes that inspired me." It's nice, but there is no longer the practically palpable relationship between the eyes and the words the man writes. What it comes down to is this: my English translation uses different words in its dynamic translation of the French.
Now, you might not think this is a "big deal", but what did Cabrel choose as the title of the song? L'encre de tes yeux, "The ink of your eyes". While I don't mind translating it for someone so they know what he's saying, I think this song in English would either sound stilted ("the ink of your eyes" does not have the same beauty in English as in French) or else would lose something inherent to the French words Cabrel chose to use.
Which leads me to Latin. (If you don't know what the post's title means, Google it.)
I've learned recently, through various sources (Adoremus, Fr. Z, and the efforts at producing a new English translation of the Latin text of the Mass of Paul VI), that the current ICEL translation of the Mass of Paul VI is pretty awful at times. The beauty is often obscured or lost, and particular Christological language is at times avoided. In addition, liturgical Latin is often very dense: a lot of meaning in a few words. This is not easily duplicated in English, which often leads to verbosity that still somehow misses the mark.
A simple example is the Gloria Patri prayer, common in the Liturgy of the Hours. The Latin version is Gloria Patri et Filii et Spiritui Sancti. Sicut erat in principio et nunc et semper et in saecula saeculorum. I've heard this translated into English in two different ways. The way I use (because it's found in the English translation of the Liturgy of the Hours I use) is "Glory to the Father and to the Son and to the Holy Spirit. As it was in the beginning, is now, and will be forever." The other way I've heard it is "Glory be to the Father and to the Son and to the Holy Spirit. As it was in the beginning, is now, and ever shall be, world without end." See, if we just said it in Latin, we'd all be saying the same thing, regardless of how people contend in saecula saeculorum should be translated (I happen to like "unto ages of ages" as I heard in a Divine Liturgy); as it is, people speaking it in English might get flustered as some of them use the first translation and some of them use the second.
Translation almost always loses something. An English translation of the Latin will always be, in some way, different from the Latin; but it doesn't have to be inferior. The more faithfully the Latin is translated, the more faithfully the Mass will be spoken by the faithful. If Latin is used instead of translating (but certainly not without knowledge of what the Latin means -- you know, proper catechesis) then what is being said and what is meant are identical. Lex orandi, lex credendi. In other words, as we pray, so we believe. (Or, literally, "the law of prayer [is] the law of belief".)