The code that Gilbert Ryle is referring to in the quotation above is culture. Just as we learn to communicate verbally, we also learn the meaning of countless strange gestures that have significance to the people of our shared culture. And to be recognized as a competent member of a community, we must master the correct interpretation and usage of those signs within that cultural context. Not knowing how to interpret them means that we will have difficulty communicating and understanding effectively even if we speak the target language fluently.
For someone crossing over into an unknown culture, this is yet another area to learn on top of an already daunting list of culture specific subjects (language, manners, proper behaviour). It may even be harder to learn than language. Language schools are everywhere; finding a serious course on "Advanced Non-verbal Communication the French Way" might be a bit difficult. In the end most of us just muddle through, making mistakes and learning through feedback (mostly negative) from natives.
Fortunately, there is some very interesting research to read on this and other cross-cultural adaptation issues. Andrew Molinsky is a Professor at the Brandeis University International School of Business. In addition to teaching a class about cross-cultural management, he does research in the area of what he calls cross-cultural code-switching: "the act of purposefully modifying one’s behavior, in a specific interaction in a foreign setting, to accommodate different cultural norms for appropriate behavior." There are two articles by him (or co-authored by him) available on-line which I highly recommend:
CROSS-CULTURAL CODE-SWITCHING: the psychological challenges of adapting behavior in foreign cultural interactions (2007)
CRACKING THE NONVERBAL CODE: Intercultural Competence and Gesture Recognition Across Cultures (2005)
Both articles are written by an academic for academics but the contents and ideas merit wider diffusion and would be very useful for anyone struggling to navigate in a cross-cultural world.