Will this be solved by the iframe JS injection?

Reply to comment #2, which is specific to Chrome for iOS:
This bug is not specific to iOS. Chrome does not translate iframes, that's a cross platform behavior. It's also unclear how Chrome should translate iframes given the current UI, where the user has to choose source and target translation language for the main frame.

Yana, do you have any comments for this bug?

Yep. In a couple of discussions that we've had, as we think about solutions for this, we may want to special-case AMP translations (vs. all pages with iframes) so that we don't need to deal with the UI challenges of content in multiple source languages.

Since the iframe is the entire page in the AMP scenario, from the user perspective we should aim for an identical experience to a non-AMP page: use the same UI but inject the translate script into the frame. 

 Issue 814279  has been merged into this issue.

This would be important to support soon, especially since manual translation is being rolled out. What's the timeline for supporting this?