All-in-all : almost on par with english, still wider spread due to the british empire and the huge US ... though depends on post-war politics of the US ITTL (isolationism ? internationalism ?).
(timeframe assumed : 20-40 years, about)
It will expand its role as 'lingua franca' in eastern europe it was short of already prior to the Great War. With lots of 'interested' students comming to germany and therefore learning, training, using the language ... and therefore spreading german language, literatur and culture also in their 'homelands' (what might include the political background of this word).
If someone wants to become someone east of the Vogese he has to use german in whatever profession.
A huge boost - in term of worldwide spreading - might become better/intensified relations towards China (depending on things go there).
It will become at least on par with english regards science, if not the leading language, esp. in 'technical' sciences.
However, which way 'german culture' goes ITTL ...
What happened to it after the Great War IOTL - what I personally regard as a huge step ... 'forward' - was to a very large degree due to loosing and the following 'loose of control' of about everything. The miseries, the uproars, the famine, all this formed the Weimar Republics culture to a large extent.
In film i.e. it - together with the amounts of workless availabe for comparsing - made a Fritz Lang (Metropolis, Dr. Caligari) possible, also determinung its 'dark' pictur language.The whole of the 'Berlin secession' artists, the burst of DADA, almost the whole of 'Modernity' ("die Moderne")wouldn't have - most likely - not taken place (gladly most likely also not Schönergs 12-tone technique for music).
IMO ITTL it would stay much more 'conservative' for quite a time, trying to fight off developments mentioned above, which might then happen elasewhere (most likely in beaten, rubbeled, revolutionary France).