TSTrader® provides the ability to change the language directly from the login page.
To update this go to the control center in the NinjaTrader platform dashboard.
Select - Tools > Options
The ability to change the language will be shown as in the screenshot below.
Please follow the steps linked here.
Tradestation does not have the ability to change the default language at this time.
All other platforms
Please email the platform provider directly to see how to change the default language.