When setup a project Lokalize ask me for "Zielsprache" (target language). That doesn't make sense to me. When translating a software project there are multiple target languages. Most of the time the base language (from your source files) is English. Then you want to translat that English into multiple different languages, the target languages. So why is the user asked for a single target language?
Lokalize is an editor for PO files, and PO files only have a single target language. See the documentation on the PO format here: https://www.gnu.org/software/gettext/manual/gettext.html#PO-Files If you would like translate an application into multiple languages, you will have a separate Lokalize project for each language. Typically, there will be different settings for each project, e.g., a language-specific glossary and a language-specific mailing list.