The full contents of the bind-variable are not displayed in SQL Plus

NLS_LANG can't be changed from inside a session, however other settings can.

You can't change the character set once a database connection has been established (ie: the 2nd part of NLS_LANG), but you can change the language with:

alter session set NLS_LANGUAGE=SPANISH

... and the territory with:

alter session set NLS_TERRITORY=SPAIN

Here's an example of how you can do this in shell script (bash or ksh for instance). Here's simple SQL script to test, it turns off (most?) of the usually undesired output from SQL*Plus:

set feedback off
set echo off
set heading off
set pages 0
whenever sqlerror exit 4
whenever oserror exit 8
select status from v$instance;
exit

You can run it like this, assuming your environment is set up properly (-s suppresses the banners and prompts):

result=$(sqlplus -s user/pass @script.sql)
if [ $? -eq 0 ] ; then
  # the script worked
  echo "$result"
else
  # something went wrong
  ...
fi

If you want to process multiple lines, you can use a while/read loop, with process substitution. Something like:

while read -r line ; do
   echo "Got: $line"
done < <(sqlplus -s user/pass @script.sql)

Or you can just redirect the output of SQL*Plus to a text file and process that later if it's not convenient in a variable.

On Windows, with cmd.exe scripts, it's all a bit harder, but you can use variants of what is described in this question: Setting a variable from an executable. Something like (not tested):

for /f "tokens=*" %%a in ('sqlplus -s user/pass @script.sql') do set RESULT=%%a

or

for /f "tokens=*" %%a in ('sqlplus -s user/pass @script.sql') do (
  @rem process one line at a time
  echo "%%a"
)

But you might be better off using PowerShell these days. (Which I know nothing about, but a quick google seems to indicate that there are good options for this.)