The index.html file sources a file with translated phrases.
Adding a new language one can simply make a copy of index.html and change its content to source another file with translated phrases.
Here is an extract from /usr/share/nginx/html/rd/index.html
<!DOCTYPE HTML>
<html manifest="">
<head>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">
<title>Rd</title>
<!-- The singletons file is the one that needs to be tweaked -->
<script type="text/javascript" charset="utf-8" src="resources/js/singletons.js"></script>
<script type="text/javascript" charset="utf-8" src="resources/js/config.js"></script>
#Create a copy of the singletons.js file and modify it for your language.
cp /usr/share/nginx/html/rd/resources/js/singletons.js /usr/share/nginx/html/rd/resources/js/singletons_de.js
#After you modified all the strings also copy it to the optimised JavaScript Code's folder
cp /usr/share/nginx/html/rd/resources/js/singletons_de.js /usr/share/nginx/html/rd/build/production/Rd/resources/js/
#Make a Copy if the index.html to indicate the language
cp /usr/share/nginx/html/rd/index.html /usr/share/nginx/html/rd/index_de.html
#Change the index_de.html file so that it sources the singletons_de.js file
#Save it
#If you use the optimised JavaScript code (which you actually should) copy also
cp /usr/share/nginx/html/rd/build/production/Rd/index.html /usr/share/nginx/html/rd/build/production/Rd/index_de.html
#Change the index_de.html file so that it sources the singletons_de.js file
#Save it
#