Even before ISO 6937 was published, a standard was approved that practises decomposition of letters into "constituents". As such basic letters and "additions" were classified. What made an addition was rather arbitrary. The dot above the i or j was never considered a diacritic, but strokes through letters were. The object group was the library world, and their lists of booktitles. Lists of that kind are being exchanged regularly by magnetic tape. ISO 5426 defines a large number of diacritics, to be used with letters in arbitrary combination, which requires special hardware at processing. Unfortunately, these things are rarely found outside libraries. Still worse, in several countries variants of their own were introduced (in so-called MARC systems), with the result that ISO 5426 and its colleagues (5427, 5428, 6861 etc.) have hardly contributed anything to international standardization.
ISO 5426 and the other bibliographic coded character sets have been developed by ISO TC 46/SC4. At a summary look to one of the series, it is clear how much lower the technical quality is of these standards compared with those from JTC1. Unclarities, ambiguities and badly chosen wording abound. To which extent ISO 5426 ever has been used in the Netherlands could not be verified with any University Library.
One should therefore realize well that referring to an implementation of ISO 5426 is pointing to a dead alley. Nevertheless, one should be prepared to the fact that many bibliographical systems are being procured as a closed package in the US, in which perhaps an implementation of ISO 5426 has been built in. Lists of titles and catalogs thus coded cannot be processed on the usual equipment.