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If you're offline, this will need to be one of the languages that is downloaded to your device. Downloaded languages have a checkmark next to them.  This will also need to be a language that is available offline. English is available offline by default.

Summary:
Open the Google Translate app. Tap the language in the upper-left corner. Tap the language you want to translate from. Tap the language in the upper-right corner. Tap the language that you want to translate to. Type or paste text to be translated. Tap the microphone to speak and translate. Tap the camera button to scan and translate written text. Tap the squiggly line button to draw and translate characters.