We have an environment that features Exchange Server systems in both the United States and Germany. We've noticed that sometimes hotfixes become available in English before they become available in German. Can we apply the English version to all our servers, both US and German?

Hotfixes typically aren't localized. Rather than taking the time to localize event log messages, UI display strings, and other language-specific components for every hotfix it releases, Microsoft tends to focus on localizing those components as part of its language-specific service-pack releases. Installing English hotfixes on a German server won't cause any harm, although you should be prepared to see unexpected bits of English in the German servers' log files.