Actually, in it's time S5 was very good. I loved to program in STL (fb's were only STL), if you got the right manuals the documentation (providing you were used to Assembler type language) were pretty good, what you could do was impressive. I think in STL Siemens had it, i.e. the use of brackets rather than the Japanese way of MPP, MPS OR LD etc. You are right though it's a cultural thing, also what you are used to, In the late 70's - 80's most of my work was on Siemens although did many others like AB, SqD, Omron Modicon etc.
Obviously documentation is always a problem in Germany there are many dialects, also something called technical German, even native german speakers did not understand that, I worked with many German people & got the impression that some technical german words did not actually exist, even people fluent in technical german struggled as there seems to be a culture of making words up if there is no direct translation for a particular way of explaining some code, one thing I remembered is a german word for floppy disk this was documented as something like floppyterian but was a little longer the actual translation was diskette in normal german. I worked with many german engineers, having conversations with them regarding some of the documentation on existing systems it became apparent that they struggled to interpret some of them one guy explained (or tried to) that often words are made up (extended) to explain an operation, this differs from engineer to engineer.
Asian documentation translations are definitely very poor, however, they are getting better.
As for IEC, if you read the documentation it always refers to function blocks so RW decided to call theirs AOI's, not that it really matters & I suppose how they are implemented may be a little different but I suppose its the same as most PLC mfgr's use N/O N/C whereas RW use XIC & XIO. how many times on here have you seen questions about that.