The interface is available in English, Dutch, German, Norwegian and Spanish.
Setting it
In the setup. It asks for a language first, and the program starts in that same language. Running the setup again in another language switches the program over as well. An update started from Settings → Info runs in the program’s own language, so it changes nothing.
Afterwards: Settings → General → Language. That choice stays until the next time someone runs the setup.
Most of the interface changes at once. A few texts are built only when the program starts and keep the old language until you restart it.
What the setting belongs to
The machine, not the rundown and not the user. Opening a list somebody else made does not change your language, and neither does logging in as a different Windows user on the same machine — the setting lives in the shared configuration.
For a building where one operator prefers Dutch and another English, that is worth knowing: it is one setting for the machine, and whoever changes it changes it for everybody.
What is not translated
Your own content. Titles of cues, names of groups, the text on your slides — those are yours and the program never touches them.
What a module supplies. A module brings its own texts in its own languages. Where it lacks one, it falls back to English, like the program.
The log. Not translated; much of it is in Dutch, which is fine — it is read by whoever supports the installation.
If a text looks wrong
The program falls back to English for anything a language does not have, so an English phrase in an otherwise Dutch interface means that text has no translation yet rather than that something is broken.
Worth reporting: which language you were in and what the screen said. Those two are enough to find it.