The lyrics are in Creole French. It is one of the many NIN songs that includes the phrase "Nothing Can Stop Me Now," and one of the few to include a vocalist other than Trent Reznor. In addition, it was the first instrumental written for The Fragile. 
The song itself was conceived during a very dark period in Reznor's life. When on stage in 2009 he disclosed that he had rented a house at the time on an ocean with the external intent to write some music, but in reality had contemplated suicide there. This song was the only song to come out of that time and place. He still feels strange playing it live because it serves as a reminder of this dark period in his life.
It can be argued that the song is heavily infuenced by the Debussy piece of the same name. Both songs build up to create a sonic atmosphere, and both are (obviously) titled the same.
When asked about any classical composers who have inspired him, Trent said in a Yahoo! Chat in December 1999:
"Debussey. I was sitting in Big Sur, losing my mind. His contribution was that you need to be regimented, with whatever you decide to do. When I was sitting there, Debussey and I had a mind meld for a second. I felt the need to plagiarize the title, 'La Mer'. He revolutionized the Mozart bullshit world of everything was rigid and stabilized."
This is the original album version. It begins with piano chords accompanied by standup bass and is then accentuated by drums playing a jazz rhythm and e-bow guitar. This is one of the few songs by Nine Inch Nails that is played with almost all acoustic instruments. It consists of a 3/4 time signature superimposed over a 4/4 time signature. Its piano theme is also used at the end of "We're in This Together."
On The Fragile vinyl, the introductory piano chords and standup bass are repeated one extra time before the solo begins over top, and the ending denouement is slightly longer, featuring more spoken samples. Because of this, the transition into "The Great Below" is also slightly different than the CD version.
La Mer (Alt)
Released exclusively through Apple Music. Part of The Fragile Instrumental. This is a significantly different mix of the album arrangement, with a very different drum sound and more pronounced standup bass. It appears to be marred with brief noise during the intro, potentially caused by a poor DAT transfer. The ending does not fade out or crossfade with "The Great Below."
La Mer (Version)
An nin.com exclusive from The Fragile era, this remix by Jerome Dillon and Keith Hillebrandt features much more percussion-oriented loops, using atmospheric and base elements from the original. The track is currently available for download at the remix.nin.com website.
La Mer (And All That Could Have Been)
This live version does not feature the speech samples, and Robin Finck plays the introductory melody normally played on piano with a Martin Backpacker acoustic guitar.
On And All That Could Have Been, "La Mer" is performed live against the backdrop of three visual screens that display images of the sea. Towards the end of the song, Reznor presses against the screen on the center and moves his hands across the panel, as the blurry image of a woman is revealed before the screen fades to black.
Frequently since the With Teeth: Summer Amphitheater Tour, Reznor has played a piano-only version of "La Mer," including an initial improvisation, before playing "Into the Void," and it is sometimes confused with "Adrift And At Peace." The band have recently begun playing "La Mer" in full again.
Et il est un jour arrivé Marteler le ciel Et marteler la mer Et la mer avait embrassé moi Et la délivré moi de ma caille À rien ne peut m'arrêter maintenant
And when the day arrives I'll become the sky And I'll become the sea And the sea will come to kiss me For I am going Home Nothing can stop me now
There is an alternate, more literal translation of the lyrics:
And the day has arrived To thresh the sky And to thresh the sea And the sea has embraced me And it has dispensed me from my cell Nothing can stop me now