The Army of the Austro-Hungarian Empire was split into three parts; only one of which would be strongly linguistically diverse.

First you have the Imperial-Royal Landwehr drawn from German speaking regions, and then you have the Royal Hungarian Honved (usually just called the Honved), which was drawn from the Hungarian side (and also included Croatian units). These parts of the K.u.K army were relatively linguistically homogeneous.

Then we have Common Army which is everyone else. A-H had conscription form 1866 for a period of 2 years frontline (12 total).

German was the 'official' language of command and control, and the average solider learned 100 words of German to understand his officers. (only 28% of the population was Austro-German).

The regiments then had a 'regimental' language; the language the majority of the soldiers spoke. Officers had to learn this language.

Overall, this didn't exactly make for an easy time commanding troops. However the linguistics were less of a concern than the rampant nationalism that would plague the K.u.K armies at the end of the war.

Sources:

Osprey Men at Arms Vol The Austro Hungarian forces of WWI vols. 1 & 2, Jung, Peter

The First World War and the End of The Habsburg Monarchy, Rauchensteiner, M