Yay, multilingualism and Austria-Hungary, as if you wrote this question just for me :)

If you were a German working with the Habsburg army, many indeed thought it was an impossible project and a testament to the overall deficiency of their partner army. Gehard Velburg, a German officer, recalled in his 1930 memoir Die rumänische Etappe: Der Weltkrieg, wie ich ihn sah that it was necessary when making a phone call "first to check what language the [member of the] partner [army] understand". "Sometimes the conversation happens in Germany, but when it is spoken too badly, you check what's the other person's mother tongue to see if you understand it perhaps".

In the army itself -- a great symbol for unity in the Double Monarchy -- three languages were spoken. A Kommandosprache of 80 or so commands persisted entirely of German. The army institutions, that is as coordinating agencies, also spoke German, which was their Dienstsprache 'service language'. Each recruit, though, had the legal right to be trained and to serve in his own language, which had to be one of a dozen or so recognized "regimental languages". On this, since I supposed you read German, see Tamara Scheer's article from 2014, "Die k.u.k. Regimentssprachen: Eine Institutionalisierung der Sprachenvielfalt in der Habsburgermonarchie (1867/8-1914)" in Niedhammer, Nekula, et al. (eds.), Sprache, Gesellschaft und Nation in Ostmitteleuropa. Institutionalisierung und Alltagspraxis (Göttingen: Vandenhoeck & Ruprecht, 2014), 75-92.

This required, quite understandably, to organize regiments along linguistic -- and consequently, thus, ethnic and national -- lines. The official ruling required 20% of the regiment to be speakers of a certain language for it to become a regimental language. This was especially problematic, as Scheer demonstrates, for officers who had to serve in a variety of environments. Linguistic training was offered, but as we know from our days as well, it was by no means a guarantee for success. An ultimately unsuccessful way to deal with this was to devise Armeeslawisch 'Army Slavic', a language of not more than a hundred words or so based on Czech. As Istvan Deak shows in his book Beyond Nationalism: A Social and Political History of the Habsburg Officer Corps, 1848-1918 (p. 99-100), officers generally chose to study Czech for it was easier later in communication with Slavic soldiers in general (even though perhaps Italian was more to their taste).

In war -- a topic on which you should refer to Tamara Scheer's chapter in Christophe Declercq and Julian Walker (eds.), Languages and the First World War (Palgrave: Basingstoke, 2016) -- this has caused many frustrations. One German-speaking officer, for example, was assigned to a Hungarian unit. He found it easier to speak English to his soldiers, some of whom worked in the United States before the war, whereas a German officer placed in a Czech regiment could only speak to his diary -- all the rest spoke Czech. Scheer presents myriad cases of conflict around linguistic policy during the war: On the one hand, languages like Czech were perceived as disloyal and singing patriotic songs in local language was a particularly detested act, but the same German officer saw how only singing a Czech patriotic anthem was able to motivate his soldiers. Many Czechs indeed deserted.

As historians like Tamara Scheer, Rok Stergar, and others demonstrate, this linguistic policy had some unexpected yet interesting results. One was the making of national differences permanent, as languages became a classificatory practice that created monolingual spaces. Many soldiers refused, or at least highly frowned upon, serving in their native language, as military service was a good opportunity to learn a language. In the Hungarian part of the empire, the ability to serve in Slovak or Romanian was not only a factor that supported Slovak and Romanian national claims but also the only state institution that recognized their linguistic rights in face of a "Magyarizing" policy that repressed minority languages in the Kingdom of Hungary. During the war, unfortunately, it also engendered the denouncement of entire regiments as traitors...