Icecat language is not set into system language

Have you yet performed "sudo apt-get update" and "sudo apt-get upgrade" ?

These two steps together usually cause Trisquel to download and install almost all available language support to a fresh installation of Trisquel unless one takes extra steps to avoid that result.

Alternatively, open Synaptic Package Manager, then type "language" into the search bar, then select "Search Results" and scroll down through the umpteen choices of language packs, selecting as few or as many that you do/do not want to be installed.

In my case, the task was to unselect all but my language of choice for a USB-Stick installation; your task my differ.

Afterwards, select "Apply" and Synaptic Package Manager will take it from there.

Here is the lnguage packs folder -> ftp://ftp.gnu.org/gnu/gnuzilla/38.7.1/langpacks/

I guess you want it in Spanish. Download ->
ftp://ftp.gnu.org/gnu/gnuzilla/38.7.1/langpacks/icecat-38.7.1.es-ES.langpack.xpi

and then just drag it to Icecat.

Doesn't "Language Support" (in the "System Settings") propose you the installation of the language pack?

I did:
apt-get update
apt-get upgrade
apt-get dist-upgrade

Still Icecat without system language.

When I go to language support into system settings, it says that language is not completely installed and suggests to install:
libreoffice-help-en-gb
libreoffice-l10n-en-gb
libreoffice-l10n-en-za
mythes-en-au
hunspell-en-ca

Why language is not completely installed before? (When I installed Trisquel, I choose my region language. And I have updated.)

Are you going to fix it?