Spanish translations (#6355)

* Add files via upload

* Update CRAN_Release.cmd

Spanish translation

* Change update date, etc.

* Update NEWS.md

* Update es.po

Corrected the word 'warning' that had been left escaped by a regexp (the term is not translated because it is a literal character string option).

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

.dev/CRAN_Release.cmd

@@ -18,6 +18,7 @@ Rscript -e "tools::update_pkg_po('.')"
 # 2) Open a PR with the new templates & contact the translators
 #   * zh_CN: @hongyuanjia
 #   * pt_BR: @rffontenelle
+#   * es: @rikivillalba
 ## Translators to submit commits with translations to this PR
 ##   [or perhaps, if we get several languages, each to open
 ##    its own PR and merge to main translation PR]

NEWS.md

@@ -197,7 +197,7 @@
 
 1. Fix a typo in a Mandarin translation of an error message that was hiding the actual error message, [#6172](https://github.com/Rdatatable/data.table/issues/6172). Thanks @trafficfan for the report and @MichaelChirico for the fix.
 
-2. data.table is now translated into Brazilian Portuguese (`pt_BR`) as well as Mandarin (`zh_CN`). Thanks to the [new translation team](https://github.com/orgs/Rdatatable/teams/brazil) consisting initially of @rffontenelle, @leofontenelle, and @italo-07. The team is open if you'd also like to join and support maintenance of these translations.
+2. data.table is now translated into Brazilian Portuguese ([`pt_BR`](https://github.com/orgs/Rdatatable/teams/brazil)) and Spanish ([`es`](https://github.com/orgs/Rdatatable/teams/spanish)) as well as Mandarin (`zh_CN`). Thanks to the new translation teams consisting initially of @rffontenelle, @leofontenelle, and @italo-07 for Portuguese; and @rikivallalba, @rivaquiroga, and @MaraDestefanis for Spanish. The teams are open if you'd also like to join and support maintenance of these translations.
 
 3. A more helpful error message for using `:=` inside the first argument (`i`) of `[.data.table` is now available in translation, [#6293](https://github.com/Rdatatable/data.table/issues/6293). Previously, the code to display this assumed an earlier message was printed in English. The solution is for calling `:=` directly (i.e., outside the second argument `j` of `[.data.table`) to throw an error of class `dt_invalid_let_error`. Thanks to Spanish translator @rikivillalba for spotting the issue and @MichaelChirico for the fix.