The interface language that installs by default matches the settings and locale on your computer. You can change it. Here's the information you need from Read This First, which is linked from the Getting Started tab:
During installation, PTC.Setup installs the product based on the language
and locale settings of your computer. You can select whether you want to install the Help
according to the locale settings of your computer or in all languages. Within the Help,
you can choose to change the Help language.
To run the application in a language different from your operating system locale (e.g. to
run German on a Belgian system), follow these directions.
1. Click the Windows Start button and then click Run. The Run dialog box opens.
2. In the Open box, type ”C:\Program Files\Mathcad\Mathcad Prime
2.0\MathcadPrime.exe” /culture:en-US. In this example, C is the drive
where Mathcad Prime 2.0 is installed, and en-US is the desired language and locale.
You can use the following abbreviations for other languages:
Simplified Chinese zh-CN
Traditional Chinese zh-TW
If you want Help in a different language, be sure that you have installed the other language. If you did not do so, rerun the installation and choose Custom install.
I hope this helps,