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/etc/texmf/jlatex/language.dat is in jtex-bin 2.1-2.

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The actual contents of the file can be viewed below.

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%% language.dat
%%
%% This file specifies which hyphenation patterns iniTeX should load
%% while generating a format file.
%%
%%
%% This file can contain two kinds of lines:
%% 1. Lines of the form
%% <language name> <list of pattern files to load>
%% instruct iniTeX to allocate new language and load all pattern files
%% for that language.
%% 2. Lines of the form
%% =<language name>
%% say that language name specified on this line is just a synonym for
%% the language defined by previous line.
%% Comments start with percent character.
%%
%% Note: the first defined language will be the default if you use no
%% language switching package (such as babel or polski.sty or
%% german.sty).
%% By convention the american english patterns should be the default.
%%
%% Below you'll find many example entries which you may want to
%% activate by removing the % from line beginning.  Before activating
%% any of the lines below please check if you have appropriate
%% hyphenation patterns files or iniTeX will fail to generate the
%% format. 
%%
%% If you've got here by using some configuration program you should
%% edit this file to your taste, save it and exit the editor.  Then
%% probably your configuration program will regenerate format files to
%% put your changes into effect.
%%
%%%%%%%%%%%%%%%%%%%%%%%%% BEGINNING OF DATA %%%%%%%%%%%%%%%%%%%%%%%%%

%% This line defines language `american' with original Knuth's
%% patterns: 

american hyphen.tex

%% If you use Babel you should include this line instead: 

%american ushyph1.tex

%% Please always include one of the above two lines!
%%
%% This line declares name USenglish as a synonym for american:

=USenglish

%% UK english:
%british  ukhyph.tex
%=UKenglish
=english

%catalan	cahyph.tex
%croatian	hrhyph.tex
%czech		czhyph2e.tex
%danish		dkhyphen.tex
%dutch		nehyph1.tex
%dutch		nehyph2.tex
%dutch		nehyph3.tex
%finnish	fihyph.tex
%french		frhyph.tex frhyphex.tex
%german		ghyph31.tex
%italian	ithyph.tex
%norsk		nohyph.tex
%polish		plhyph.tex
%portuges	pthyph.tex
%romanian	rohyphen.tex
%slovene	sihyph22.tex
%spanish	sphyph.tex
%swedish	sehyph.tex