I would like to translate "One Percent". As in, one percent of the
population. It will be engraved on a gift. I would like to place my
order today so a prompt response would be appreciated =)  So far I
have come up with 'Unas Centesimas' but I am not sure if that is
correct. Thank you!

Hello.

I'd go with:

Unus per centum.

The ancient Romans reportedly didn't use percentages, but the format
that we see in modern scientific literature is:
cardinal number (unus, duo, tres, etc.) + "per centum"

Examples:
"...(sex per centum) (Keely et al 1994), TATA-colligans proteinus
(novem per centum) (Sunkin and Stringer 1995), alpha-subunitas guanini
nucleotidi colligantis proteini (septendecim per centum) (Smulian et
al 1996), BiP praenuntius (septendecim per centum) (Stedman and Buck
1996), aestus-offensio proteinus septuaginta (duodeviginti per centum)
(Stedman et al 1998), internae transscriptae seper-antes regiones
intra nuclearem ribosomalem RNA locum (triginta et duo per centum)"
http://www.mycologia.org/cgi/content/full/96/3/429

sex per centum = 6%
novem per centum 9%
septendecim per centum = 17%
etc.

Also see:
"duo per centum" (2%)
://www.google.com/search?hl=en&lr=&safe=off&q=%22duo+per+centum&btnG=Search

Also see: 
Cardinal numbers - Latin:
http://www.legionxxiv.org/numerals/

--------
search strategy:
"unus per centum", "duo per centum", etc

Thanks.

I think she is not really askig from a latin translation, she wants to
know the spanish translation for "One Percent", that is "Uno por
ciento"

But the answer you gave her, I think is OK

ddeleonm,
Thank you for your comment. The title here is "English to Latin
translation."   I have to take that literally.  The customer,
jennym03, did not request Spanish.  Additionally, it is fairly common
to use Latin for engravings.
Regards,
juggler