This is a great question, and pretty well answered by available secondary sources. I am a historian (completing my PhD) studying the Prussian Army (and British Army) in this era, and will try to answer your question to the best of my ability.
At the outset, I want to state that unlike most European Armies that the time, the Prussian Army recruited among its own subjects via conscription (kind of like a "draft" lottery) called the Canton System. In 1732, Prussia adopted a form of conscription called the canton system. Prussian soldiers, as opposed to being composed only of landless men who joined the military voluntary, began to represent more of a cross-section of Prussian society, with many second sons from rural peasant families being conscripted into the military. These soldiers were what we today think of as reservists: they were called up for 2 to 3 months of the year in order to train with the army, and spent the rest of the year at home pursuing work in the civilian world. Non-cantonists, although they were not sent out into the countryside to work, spent 4 days of the week working in civilian trades, as part time laborers. As a result, the military grew, and the economy gained the benefit of numerous workers which would otherwise have been lost to the military. Although Prussian peasants were initially horrified by the canton system, desertion in the peacetime Prussian army was very low, perhaps 7-8%. (For more on the Canton System, check out Otto Buesch, Military System and Social Life in Old Regime Prussia)
Second, it was incredibly common for foreigners to join the Prussian Army, in any given year of the eighteenth-century, they made up about 30%-50% of the army. At the end of the Seven Years War in 1763 for example, they made up approximately 31% of the army. (Duffy, Army of Frederick the Great, 2nd edition, 76). Many of these "foreigners" were volunteers from within Prussia, native sons who volunteered in the Prussian Army (ie, they were not conscripted by the canton system) were also listed as "gemachte Auslaender" or "fabricated foriegners."
Turning from this slightly confusing concept to the true foreigners (soldiers recruited from outside the boundaries of Prussia), many of the actual foreigners in the army would still have spoken a form of German: they would have come from states such as Saxony, Bavaria, and Switzerland. Indeed, at the end of the Seven Years War, approximately 1 in 5 of the listed "foreigners" was Saxon in origin. (Duffy, Army of Frederick the Great, 76) As a result, the differences in language would be superficial.
The actual foreigners, alien in both language and culture in the Prussian Army, would have found a group of fellow recruits in their regiment: it would be very odd for a Prussian recruiting party to acquire only one soldier when traveling so far afield. Thus, both the narratives of Ulrich Braeker (from Switzerland) and Johann Christian Schimmel (from Saxony) seem to indicate that they retained affection for their homeland, and in Braeker's case, he generally spent time off-duty with his other Swiss countrymen in the Regiment. ( Ulrich Bräker, Lebensgeschichte Und Natürliche Abentheuer Eines Armen Mannes Von Tockenburg (Zurich: Hans Heinrich Füssli, 1789); Johann Christian Schimmel, "Kurze Lebensbeschreibung Des Preußischen Veteranen Johann Christian Schimmel.," Zeitschrift Für Kunst, Wissenschaft, Und Geschichte Des Krieges 10 (1827) )
In the case of soldiers from the British Isles, I have yet to come across any Englishmen (although I have read the letters of an English volunteer in the army opposing the Prussians, the Austrians of Maria Theresa). However, several Irish soldiers were pressed into service for Frederick William I (Frederick II "the Great"'s father), including James Kirkland and Tomás Ó Caiside. These men would have learned the German necessary for infantry drill, and likely been conversant in German after several years in the army. A number of these individuals actually tutored officers in foreign languages, likely with French as the language of instruction. (Duffy, Army of Frederick the Great, 77)
Obviously, when these men were came from affluent noble backgrounds, they were much more invested in the cause that they volunteered for. John Darrell, an Englishmen from Kent, volunteered as a junior officer in the Austrian Army of the Seven Years War, and wrote home to his familiar with an obvious interest in the war and the actions of Generals such as Loudon and Daun. (Kent Local Archives and History Centre, U386/C10) Likewise, in place names and enemy some family names, Darrell's spelling began to take on a more German character, sometimes he uses the correct German word for a town, other times he spells out a word phonetically. (Kent Local Archives and History Centre, U386 C3).
Culturally, especially if they were recruited against their will, soldiers were resentful of being enlisted in a foreign army. Marcus von Salisch has written an excellent book (tragically, it is only available in German) Treue Deserteure, which examines the great lengths that some Saxon soldiers went to in order to escape service in the Prussian Army after they had been forcibly recruited in 1756. Their efforts to demonstrate their loyalty to their own sovereign put them at odds with their new Prussian officers.
A couple of final points, all taken from the excellent work of Christopher Duffy:
Some suggested reading:
Ilya Berkovich, Motivation in War: The Experience of Common Soldiers in-Old Regime Europe
Otto Buesch, Military System and Social Life in Old Regime Prussia,
Christopher Duffy, The Military Experience in the Age of Reason
-------------------------, The Army of Frederick the Great, 2nd edition
Marcus von Salisch, Treue Deserteure
Sascha Moebius, Mehr Angst vor dem Offizier als vor dem Feind?
-----------------------, Prussian Army soldiers and the Seven Years War: The Psychology of Honour
And, if I might not be too bold, you can read my blog on common soldiers in the eighteenth century for free: https://kabinettskriege.blogspot.com/2017/07/the-average-mid-eighteenth-century.html