For projects spanning to multiple languages(more than 2 to be precise), app building might be inefficient at times with long forms displaying translations for all languages.
You can change the setting "Smart language display in forms" under languages list on app settings page. 
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Once enabled:
When selected/current language is same as the default language, only the default language is shown.
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When selected/current language is different from the default language, both default language and the selected language is shown.
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Note:
When adding new questions to the forms, translations for other languages will be stored same as the default language keeping the behaviour you would have without this setting enabled.