As we already know, WhatsApp has been working on an in-app translation feature since last year. After releasing the feature to certain beta testers, the messaging app is now making it available for general users on Android and iOS.
When viewing a message in a different language, the user can long-press it and tap the “Translate” option. The user can select the language they wish the message to be translated from or to. Aside from that, they can download the language for future translations. To view the original message, the user can simply press and hold the message again and tap “View translation”. These translation capabilities are available for all chats, including one-to-one conversations, groups, as well as Channels.

According to WhatsApp, these translations are all carried out on the user’s device “where WhatsApp cannot see them”. This is to preserve the user’s privacy, as no data is sent to any external servers.
Android users get an exclusive automatic translation functionality, allowing any future messages in a given chat thread to be translated without requiring user input. It is unclear whether this feature will remain for Android devices only, or if a future update will add it to iPhones.
As for the supported languages, Android users will first have access to a total of six: English, Spanish, Hindi, Portuguese, Russian, and Arabic. Meanwhile, iPhone users can use the feature in more than 19 languages. WhatsApp promises that more languages will be added later, although it did not mention any specifics.
Given that this feature is gradually being rolled out, not everyone will immediately have access to it. WhatsApp advises users to keep the app updated so they can access message translation as soon as it’s available.
(Source: WhatsApp [1], [2])