I believe that the main assumption they can make is that they can use the name on the ID forms issued by the government or, in case of foreign citizens, their p

I believe that the main assumption they can make is that they can use the name on the ID forms issued by the government or, in case of foreign citizens, their passports. Due to history of international diplomacy, the general standard for passports expects that in addition to whatever script the country uses, they will also include the name of the person in English or French - so this is the key source of the problem, as for passports in e.g. Russian you will get an "English" name that you might use, however, you may get passports with names only in French, so you would have to support the English and French alphabets but perhaps not necessarily any others. Regarding trailing spaces etc, IMHO the standard would be "as shown in passport" i.e. trailing spaces definitely would not matter, but spaces and punctuation between words would (e.g. D'Artagnan as a name). I looked for but did not find any specific restrictions on name length. In general, the country will have regulations on what they accept as names in their official IDs, and again you may piggyback on other institutions - as long as you accept everything for which your government have issued documents, you should be fine; and if someone has an interesting case that requires changing the process, let that fight happen between them and the government first.