Not really a problem, but I want to understand: What exactly does it do?
Currently icedove-l10n-de would go away during d-u (due to not being compatible with the new icedove package).
I am able to work with an english GUI but having it in german doesn't hurt ;-) .
From "man apt-mark" I only learned how "manual" affects the removal of a package.
But when marked "manual (-ly installed)" will this package come back without user intervention during a following d-u when it gets available in a compatible version?
I know I could "hold" it using apt-mark (and not install the new icedove version), but then I would need to unhold it during the next d-u only for looking if is available or not.
Don't know. But you can test it by yourself without getting in trouble because the icedove-l10n-de in experimental works with icedove in unstable.