Journey Language is a setting that allows you to take users on a journey only if their languages are the same as what you select in the launch settings. The All Languages option is selected by default. You can change it based on the language on your website or mobile application. 
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If you pass data from your tool to Insider via Upsert User Data API and do not send the “language” information of your users, you need to select All Languages.
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If you select a language for which a user does not have a language attribute, this user cannot enter the journey.
Once you select your language, proceed with other journey launch settings.