If the current forum language does not exist in the plugin, then overriding strings in the admin panel does not work.
As an example: if I have a plugin which only has english language translations, and somebody installs it on a french forum, the english strings will be used (this makes sense).
However, if the forum owner goes into the admin panel and tries to update the translation string, that change has no effect.
And also in April:
Could be related to this: