what is the difference between ISO, ISA, DIN and EN standards

The most appropriate codes and standards is usually a function of what you are specifying. Ultimately you, or your customer, have to decide on the governing standard.

I am on boards for ASME and ISO with respect to flow measurement. ASME (like the rest of the world) is leaning toward standards reconcilliation with ISO, however valid differences still occur.

A lot of good research is coming out of Europe, hence I am biased toward ISO. ANSI, DIN, and BS (British Standards) have formed the basis of many ISO standards.

There are also AGA (American Gas Association) and API (American Petroleum Institute) standards that tend to steer their own path, largely independent of ISO and others.

ISA, of which I am a member, is, in my opinion, becoming either dated or superfluous with respect to codes and standards. I don't mean to unfairly knock the organization, and some of the work coming out of ISA is timely and of high quality. Generally, I see ISA as a declining factor.

Sorry if that offends anybody.