Therefore, the Windows operating system I use is Windows 7 Ultimate. With this operating system, I can activate any of the languages supported by Microsoft after downloading (for free) and installing them. Each user login I create can be configured to use a separate operating system (and it has its own Desktop, so I can avoid showing off my usual clutter of icons in my working profile).
Simply by switching the login, I can toggle between German, English, Russian and Portuguese systems quickly.
My primary working environment for translation, memoQ, also supports a number of user interface languages of interest to me and my clients. To change the language for the dialogs and menus in memoQ, I go to Tools > Options... Appearance. The dropdown field at the top of the dialog shows the available languages in the version of memoQ you use. Here is the selection for memoQ 2013:
To see the new language chosen for memoQ, you must quit the application and restart it, unlike other settings changes possible for the appearance, which become active as soon as OK or Apply is clicked.