Junior professorship (W1) in Germany without full German proficiency

You can certainly apply. I don't think German language proficiency will have a major impact on your chances, as long as your research record/profile is good and you are able to at least understand some German in case a student asks in German. Computer science students usually need to be able understand at least some English...

I disagree with earlier posts that you would be "expected to give undergraduate-level lectures". On the contrary, they would usually prefer you to give smaller (=specialized) classes until you have some more experience... Sometimes, you can find this in the advertisement.

However, German academia is very competitive!

We do not have a lot of professor positions, and it appears quite common to take 6-12 months to fill a position (make that 12-24 months for full professor positions)... according to Times Higher Education, Germany has the worst student-to-professors ratio (source). You may say, the German system is very efficient. We have few professors, yet we have a very high output of very educated people!

As far as I can tell, you need to:

• be the best match for their plan
• have had some third-party funding
• have a strong track record (papers in top venues, awards, 100+ citations)
• have been at different universities
• have some teaching experience
• convince them in person

So don't be surprised if you don't hear anything back (not even an invitation to present) on your first applications. Maybe you will get a negative reply after a year or two, when the new professor has arrived at his position. Maybe not even that. So, do not wait for an answer when you apply. Continue sending applications and making backup plans.

There is a reason why Postdocs ("Mittelbau") complain a lot in Germany (according to a recent study, even 50% of Junior Professors consider quitting academia because of the lack of perspective - most junior professor positions are currently not tenure-track). They do a lot of the work (both in research and teaching) and usually have only up to three years of work contract.