I think the answer to your question might be highly dependent on the level of experience of the interviewee. You don't interview a recent grad the same way you might someone with ten or twenty years of experience.
Any decent software engineer should be able to do well with just about any language within, say, 4 to 8 weeks and rock on it in six months. These days a lot more of it is about libraries and frameworks than anything else. For example, you can be great at Python and it ill still take a major effort to get into Django, Nginx, gunicorn and figure out a sensible deployment strategy.