A bit of knowledge about the author and the history and sociology of Mauritania provides some important context. Mariem Mint Derwich was born and grew up in Mauritania, then studied and worked in France (source: Le Devoir). She considers her plural identities to be very important. She does not relate to France in the same way that she relates to Mauritania: she says she “sleeps in France, but lives in Mauritania” (source: Jeune Afrique). To put it another way, she spends most of her time in France, but she is most emotionally attached to Mauritania.
Her mother was French and the grand-daughter of a Calatan immigrant, and the author herself was “raised by a mixed Moor and Bambara” (source.) Note that Mauritania has a deeply rooted tradition of Moors enslaving darker-skinned people, so being mixed Moor-Black is highly significant.
This poem is about her attachment to Mauritania. The narrator is the author. “You” is the country, Mauritania: “my country” (or “country mine” to be closer to the French original) in the opening line is the person that the narrator is speaking to (it would be a vocative in languages that have one; French does not have a vocative, but there's no other way to interpret that clause). This is explicit in the last line: “you will become a country within”¹. It is fairly common for poems glorifying a land to be grammatically addressed to that land (for example “O, Canada”, Canadian national anthem, “Ô Belgique” (La Brabançonne, Belgian national anthem), “Douce France” (song by Charles Trénet), …).
The third person singular (“she”, “her”) is “your daughter, daughter among your daughters, daughter among your men”. This is clearly a Mauritanian woman, but not necessarily a generic or idealized Mauritanian woman: there are also references to the author, making the poem a request for the country to tell the story of the author. There are references to intimate details, most of them rather generic if intimate (“the scent of her mother”, “daughter of clouds”, “she has eyes open, your eyes”), but some more specific (“she offered her hands to the cliffs of Amogjjar”, “she is the daughter of encounters, mixed daughter”). I don't know if Amogjjar has any particular cultural or personal significance. “Mixed daughter” has a clear personal significance: it is a reference to the author's plural identities. Having at least French, Moorish and Bambara roots, the author is a “daughter of encounters” in a way most people are not. On the other hand, near the end, she writes “you will tell them, her people, that she sleeps under cemetery stones, in the prayer of those who rest and hope”. This is clearly not a reference to the author. My interpretation — but at this point it is a subjective interpretation, not something I can unambiguously deduce from the text and context — is that the author is projecting her own plural identities onto all Mauritanians, and Mauritanian women in particular, in a message of tolerance and plurality: there isn't a single Mauritanian identity, every Mauritanian should accept other ethnicities and cultures as equally Mauritanian. This would be consistent with the author's message of religious tolerance (Le Devoir), in favor of freedom for women (Adrad Info), and in favor of multiculturality (Moor, subsaharan and French):
If I had one priority, it would be for Mauritania to accept its multiculturality, its role in a Subsaharan and Maghrebi Africa that has eyes set upon it. And France has been there too. Colonization is over, but the legacy is there. (in Jeune Afrique)
As for “them”, there are a few places where this is more explicit:
You will say to them, to your people,
you will tell them, her people,
You will inscribe in their blazing gazes, your name,
“Them” is the people of Mauritania, both described as “your people” and “her people” (again highlighting how the author's own diversity should be considered an intrinsic part of the Mauritanian identity).
¹ The original French is “tu deviendras pays intime”, literally “you will become [an] intimate country”. The connotations of intime are fairly similar to the English “intimate”, and I don't get the English translation. Mind you, I don't really understand the original either.