To customize Google Translate to use informal language when translating text, you can try using informal pronouns or adding informal phrases to the text you want to translate. For example, in English, using you instead of you all or thou instead of you can sometimes result in an informal translation. However, it's important to note that since Google Translate is based on statistical inference and not structural parsing substitution, the usage of informal language can be inconsistent.
If you are translating from English into languages that distinguish between formal and informal forms, it can be challenging to get the app to accurately distinguish between the two. In these cases, adding a personal name or informal phrase to the text can sometimes force Google Translate to give you the informal form. However, this method may not always work, and the accuracy of the translation may still vary.