This is probably not the forum, but who do I contact to improve DT translations?
You can improve translations yourself. Check out the source code, find the .po file for your language, open that file in POEdit, edit it, save it, and make a Pull Request on github.
Hello, translators often work many years on a project, so instead of directly improving an existing translation, it’s perhaps better and certainly more polite to contact the translator of the language in question. Perhaps she is just working on improvements! Click on the Darktable logo on the top, left, to find their names.