i think in addition to being a language druidic should function as a sort of permanent speak with plants/animals, maybe requiring a low skill check to translate, (and make sure rangers get the choice to learn it if they don't already).
however druidic would only available to take through feat or by something from one of your species/class/background explicitly granting you the ability to take it, as it stands it's so niche, theives cant is at least more widely applicable to be known and i think is sometimes used as a de-facto sign language which gives it use in stealth situations.
the speak with plants/animals spells are the sort of thing that i think the wizard ought to be able to know, but the druid and ranger should just be able to do, the wizard takes it weighing it as a risk niche spell they may not even have a chance to use, the druid and ranger can talk to every cat, dog, bird, bush and tree as they wish and it's a resource.