 <blockquote> <p>So my question is if translating a library into another language makes it “derivative work”</p> </blockquote> <p>Yes.</p> <blockquote> <p>and if so is it enough to open-source just the library, or must the entire application using it be published under LGPL as well?</p> </blockquote> <p>If it is a good-faith translation (i.e. without any other changes), only the library needs to be open-sourced.</p> <blockquote> <p>In the mean time the license was changed to LGPL (for the original JavaScript version), but does this affect the translated versions or are they still pure GPL?</p> </blockquote> <p>Any translation or other derivative work based solely on version X of a software is not affected by the licenses of any other versions of the software, earlier or later. So yes they would still be GPL, unless explicitly changed.</p> <p>This is not legal advice and I'm not a lawyer.</p>