Does a 303 pass PageRank like a 301?
I don't know, but I can devise an approach to your problem based on what I do know. Specifically:
- I know from Google sources that a 301 will pass PageRank and anchor text
- For the purposes of getting a user from
example.com/en/, both 301 and 303 give the same result
So should you recommend switching it?
If passing PageRank is a concern, there's a good argument to use a 301 instead: we know it passes PageRank, and using it doesn't harm functionality, whereas we don't know what Google will do with a 303.
Also, a lot of search engine behaviour can be inferred from the specifications of the technology they use and work with. The specification for the 303 states that "The new URI is not a substitute reference for the originally requested resource". So the destination of the redirect is not a substitute for the original: that suggests to me that a search engine might treat it more like a temporary redirection than a permanent one.
When should you use a 303?
The specification states that it's intended for forms, which is a very specific intention. I certainly can't think of any other uses that would fit the crux of that specification (quoted above).