Is there possible to translate the buttons and elements of the PDF Light Viewer interface in WordPress ? Throught WPML or directly in the code ?
You definitely can translate it using WordPress translation files (POT files). The base version contains the following languages: https://github.com/antongorodezkiy/pdf-light-viewer/tree/master/languages
PRO Addon also have translation files in it’s “languages” directory.
Unfortunately I cannot say for sure about WPML, quick search shows that people ask questions about it. Though we’re using wordpress built-in translation function, so I presume it may work.