The order of places 1–2 and of places 4–5 might switch places; and python might drop down to place 5, depending on how you count.
Java is the standard language for serving applications in Google. When you write an application server – something that handles incoming user traffic – you are typically choosing Java as your language of choice. And Google has a lot of those.
C++ is the language in which most of our system and deep backend code is written. C++ is typically the language of choice for programs where you will interact with lower-level system interfaces, either because your code does something low-level (like, say, filesystem management), or because your code will do non-standard things because you really want to optimize every CPU cycle.
These two languages probably cover the majority of the code written at Google.
Python is the standard tooling language – most engineers at Google use some Python, but it’s not the main language for them; for writing a little tool for some processing of data or automating some manual steps. Some of these tools grow up and become larger systems, and while people usually would prefer to rewrite them to a strongly typed language at that point, they frequently don’t find the time for it. So, Python is probably the third in terms of how often code is written in it, but since fewer actual systems are written in Python, it’s probably lower in terms of how often code is executed.
Go is the new system language of choice. New system-level code is frequently written and Go (for instance,is in Go). It’s probably higher than fifth in terms of how much new code is written in it, but most Google engineers work on pre-existing systems, and the time and risk in rewriting the codebase wouldn’t be justified – so it’s probably fifth (maybe fourth) in terms of how often engineers actually write Go code.