By default Music Speed Changer uses the device system language. It is possible to have the app use any of the other available app languages regardless of the system language. The currently available app languages are English, French, German, Japanese, Korean, Portuguese, Russian, Simplified Chinese, and Spanish.
Adding the Language You Wish to Use
If you know your device has your desired language enabled, you can skip to the next section. Else, you want to make sure the language is there, and if not, add it. To do this, tap on the Settings app on your device's Home screen.
In Settings, tap on General
Tap Language & Region
Below you see the available languages, English, French, German, Japanese, Korean, Portuguese, Russian, Simplified Chinese, and Spanish. If you do not see the language you desire from this list installed on your own device, you can add it on this screen. To do so, tap Add Language..., and select the language to add.
Your device will get the language and install it. If the language is your preferred language for your phone, you can set it as default, and the app will automatically switch to it. If not, just make sure it is available on the list of preferred languages on your phone before preceding to the next step.
Changing Music Speed's App Language
After you know your device has your desired language installed, you can choose to use it for Music Speed iOS.
In the main device settings view, scroll down to and tap on Music Speed
Tap on Language
Choose the language you want to use
That's it! Music Speed will now use the language you selected.