On a landing page for a PFX instance, you can use these parameters in links to different language versions: 

• locale – Sets the format of dates and numbers.
• uiLanguage – Sets the language of the application. If not specified, uses the language specified by locale 

As a value for both parameters, use the ISO language code.

For example, the last link on the screenshot below looks like this: https://test.pricefx.eu/priceFxWeb.html?locale=de&uiLanguage=en#loginPage

It keeps the UI in English, while dates and numbers are formatted according to German.