I am waiting for the need to translate a disabled language to become a real request. Right now everyone (3 ppl) that asked for it, asked because they think it's needed for others as opposed to actually experiencing a context where that's a need for their own experience.
Discussion
This is exactly the experience I use frequently on Twitter. I can read and write basic Japanese but I'm not good at it so I like that the UI gives you the option to request it.
For all languages or just Japanese?
For Asian languages I prefer the behavior of no auto translation but to have the button to feed a particular post into the translator. I'm not alone in this.
For Spanish I'd like the opposite. Auto translate is OK but I want the button to flip it so I maybe I can learn a word.
One of the reasons for this usage preference is because Asian language auto translation is often far worse than for European languages.