How do I decide between formal and informal address in translations? 

English hides the question. Many languages do not: German chooses between Sie and du, French between vous and tu, Spanish between usted and tú, and Japanese and Korean grade politeness through verb forms. The choice is not a translation detail, it is part of the product's voice, so decide it once and apply it everywhere.

How to decide. Think of who is talking to whom. A banking or medical product addressing adults who are not its friends is formal in most markets. A consumer app that speaks casually in English, with contractions and exclamation marks, is usually informal in German or Spanish too, because a formal translation of a casual sentence reads stiff. When in doubt, look at what competitors in that market do and at what your own support team writes to customers.

Write it down. A decision that lives in one translator's head will not survive the next translator or a batch of machine translation. Put it in the project description or style guide that every contributor sees. In Localizely that field also feeds AI Translation and the AI Assistant, so generated text follows the same register as human work.

Apply it to every string. Mixed registers are worse than either choice. A single formal error message inside an informal app stands out immediately to native speakers. Review per language with the register in mind, not per string.

Switching an existing translation. If a translation came in with the wrong register, do not rewrite it word by word: the Formal and Informal actions of the AI Assistant rewrite a translation in the other register and show the difference before you insert it.

Two traps. First, formality is not the same across languages: the Dutch u is used less than the German Sie, and Brazilian Portuguese is informal in situations where European Portuguese is not, so one global setting is a starting point, not an answer. Second, some UI text has no addressee, such as a menu label or a table column, and should not carry a form of address at all.