The international mode of the FFS changes how file names with international characters are handled. Note that FFS is not case-sensitive, that is, file name matching needs to know how to convert letters from lower case to upper case to identify a file name - and also how to compute the hash-value of a file name to quickly locate the file.
Now, the OFS originally only did that based on the ASCII encoding, i.e. it would identify a=A, but it would fail on the upper half of the ISO-Latin-1 encoding, namely ä!=Ä. So, it was case-senitive for "funny German Umlauts" to give one example. The international mode finally made the FFS "ISO-Latin" aware.
All "modern" variants of FFS are "international" in this respect, i.e. handle upper and lower case correctly and handle case-insentivitiy also for letters beyond the ASCII set. That is, the dircache, long file name and long-file-name compatibility variants of FFS and OFS are all fine in this respect.
There is really little reason - except for legacy or compatibility to 1.3 - to use the incorrect (first generation) versions.
Other than that, the administration information and block layout etc. is all the same.