Help+Manual itself is really the best XML editor for editing and translating your projects. The inexpensive Basic version costs less than most professional XML editors and can do much more. It can edit both compressed and uncompressed projects created with the Professional or Floating license version without restriction. It is just lacking some features, but they are not features the translator needs – for example ePUB and Visual Studio output. Unlike a normal XML editor, you can work in WYSIWYG mode and you also have access to all of Help+Manual's advanced tools and features.
Translating projects in Help+Manual is basically exactly the same as working on a project in your own language. You just make a sibling copy of the project with the Project Synchronization Tool, set up the language settings appropriately for the target language and work you way through the project, translating as you go. When a new version of your project becomes available you can then use the Project Synchronization tool to create an updated version of the translated project containing all the topics with new material to be translated.