Tens of thousands of people in Poland are denied that right.
well I would presume it is because of the technical problem - the machinery used to produce Polish ID's (dowód osobisty and perhaps prawo jazdy) is probably designed to use Polish sings only
I think it is pretty similar in England - they don't have any diacritic signs in their alphabet- do you think they use all those special letters from around the world issueing their documents
(well perhaps it has changed in recent times and i mean very recent)
there are some obvious obstacles for introducing the multi-language set of letters for use in Polish documents - I don't mean the Vietnamese set of letters cause I have little knowledge of it here - but look at various others - look at those c's, s's and t's with cedilles -as in Romanian or Turkish language - look at umlauts of German, Scandinavian and Hungarian or those these all present a problem for Polish people to know how to read them or even are difficult to pronounce for a Polish person - and it is not the Polish people in general but authorities like say police or branches of various administrations that would mostly be presented with the problem - are they able to cope with that yet?
do you think this problem is solved in most European countries - I am not that sure - but perhaps you are better informed than me so I very much await your insights
this all is not to say we don't need to eventually introduce the multi-language sings use in all documents issued in Poland - we will have to live with that - this world is getting more and more multicultural after all
but then again even after all this there is still a question of those non-Latin writing systems - it is pretty obvious we need to transcribe these - there are several traditional Polish transcryption set of rules for foreign non-latin alphabets (or rather languages) - like traditional Polish transcryption of Russian, Ukrainian, Belorusian and perhaps Serbian and Bulgarian (not sure about these latter) - perhaps we need to abandon them and use European-wide system based say on the English transcryption - but some compromise solutions can be found I guess - like say English transcryption for other than Slavic languages -
one can also imagine a mode of issuing documents in traditional national (Polish in case of Poland) transcryption of non-Latin alphabets(or even other writing systems) augmented with the use of the non-Latin script as used by the person in question to denote the personal data (say just the names) to make them more universal