The language used in Rocket League can be changed on each platform.
Epic Games Store
Open the Epic Games Launcher
Click on the Profile Icon (the first letter of your user name) in the right upper corner.
Select Settings.
Click on Rocket League.
Check the Additional Command Line Arguments box.
type: -language=xxx
xxx - replace this with the 3-letter code for your language
English - INT
French - FRA
Italian - ITA
German - DEU
Spanish (LATAM) - ESN
Dutch - DUT
Brazilian Portuguese - PTB
Japanese - JPN
Korean - KOR
Turkish - TRK
Russian - RUS
Polish - POL
Playstation
Go to your Playstation Dashboard.
Select Settings (located on the top right).
Select System Language.
Select your desired language.
Xbox
Press the Guide button on your controller.
Go to Settings.
Select System Settings.
Select Console Settings.
Select Language and Locale.
Select Language.
Select your desired language.
Nintendo Switch
Go to the Home Menu.
Select System Settings.
Select System.
Select Language.
Select your desired language.
Steam
Click Library.
Find Rocket League in the left-hand column list.
Right-click Rocket League.
Select Properties.
Select the Language tab.
Select the language from the drop-down menu.