Refer to the Prime Installation and administration guide. There you will find something like (This is for Prime 4):
"Running PTC Mathcad and PTC Mathcad Help Center in Other Languages
You can run PTC Mathcad Prime and PTC Mathcad Prime Help Center in the following languages: English, French, German, Italian, Spanish, Japanese, Korean, Simplified Chinese, Traditional Chinese, and Russian.
Running PTC Mathcad in Other Languages
This section provides information about running PTC Mathcad Prime in a language different from your operating system locale.
1. Click the Windows Start button and then click Run. The Run dialog box opens.
2. In the Open box, type "C:\Program Files\PTC\Mathcad Prime 4.0\MathcadPrime.exe" /culture:en-US. In this example, C is the
drive where PTC Mathcad Prime is installed, and en-US is the desired language and locale.
You can use the following command arguments for other languages:
Language Command Argument
Simplified Chinese zh-CN
Traditional Chinese zh-TW"
Note that you can also (try to) switch to e.g. Brasilian Portuguese by putting pt-BR after the /culture: switch. Prime will not issue an error, but failing to be equipped with that language, will use English, although the locale info says its pt-BR...