[Koha] Adding languages to Advanced Search

[Rubén Fernández Asensio]

Hello again!
I'm still trying to customize the Advanced Search.
Now I need to add a language to the language search field. For some 
reason, it doesn't show Esperanto ("epo" according to ISO-639-2).

Taking a look at file 
/usr/share/koha/opac/htdocs/opac-tmpl/bootstrap/en/modules/opac-advsearch.tt, 
which seems to govern Advanced Search interface, I find this:

<fieldset>
   <legend>Language</legend>
     <!-- LANGUAGE LIMIT -->
       <p>
	 <label for="language-limit">Language: </label>
          <select id="language-limit" name="limit">
          	<option value="">No limit</option>
                    [% FOREACH search_languages_loo IN 
search_languages_loop %] [% IF ( search_languages_loo.selected ) %]
                 <option value="ln,rtrn:[% 
search_languages_loo.iso639_2_code %]" selected="selected">[% 
search_languages_loo.language_description %]</option>
                                                 [% ELSE %]
                 <option value="ln,rtrn:[% 
search_languages_loo.iso639_2_code %]">[% 
search_languages_loo.language_description %]</option>
                      [% END %]
                                             [% END %]
           </select>
        </p>
      <!-- /LANGUAGE LIMIT -->
</fieldset>

So, what should I do to add a language and its description to the list, 
or at least to take a look at the built-in list "search_languages_loop"?

Also, is there any way of writing code to select "any language but X"?

Rubén