the Austrian empire was pretty diverse. So WI in order to try reduce ethnic tensions, somehow a language was made from mixing together German, Hungarian, Czech, Polish, Croatian, Romanian , etc. And this language is used as the official language. How would this effect ethnic relations within the empire, and the nation as a whole.
Depends on when it happens, but in the most likely timeframe, the most likely result would be such that "backfire horribly" is an understatement.
People were
attached to their languages, and having to learn a new one imposed from at what would be perceived as the whim of the court would go down rather badly.
You can pull off this sort of trick, to a point, with closely related linguistic forms, or varieties to which there is very little identity value attached (easier if it's both). However, Hungarian, German, Romanian and the several Slavic languages of the Empire are
very different to each other (Hungarian in particular is quite unlike anything else around) in grammatical structure and a big part of lexicon (lexicon is probably the bit where resemblances are more common at first sight, because of several layers of shared loans). Also, several of these languages had already established written standards, prestige, literary tradition and connections, whatever imagined or creatively reconstructed, to historical political entities (unless you see this happening before c. 1800, which strikes me as odd). You could probably define a compromise standard between Czech, Slovak, Polish, Ukrainian, Slovenian, and the Serbo-Croatian dialect continuum; some varieties would still be marginalized. But it is hard-to-impossible to fit German, Romanian and, worse, Hungarian in it. I mean, sure you can get linguists create a conlang from that. But having people actually
using it, or accepting it... unlikely in the extreme, and very likely to rub a lot of people exactly the wrong way.
You might have a slightly better chance (that is, better than a snowball's in Hell) is the Austrian government, in a fit of utopian creativity, decides to go for experimenting Esperanto*. Pity that, by the time Esperanto existed, the Austrian government had not power whatsoever to enforce it in Hungary.
* I can actually see the Badeni or Taaffe suggesting it, in a fit of frustration at the endless linguistic bickering, in Bohemia and would-be Slovenia in the 1890s. It would be in character for Empress Elisabeth to support it (not that it matters much at the time, she was essentially estranged from the court). Won't work of course, but the idea is fun.
