Open the Account menu and choose Language, or go to Settings & billing and pick one under Language. You can also pick one on the login screen.
actuals speaks English, Español, Français, Deutsch, Português (Brasil), العربية, 简体中文, हिन्दी and 日本語. Languages marked Beta have not yet been checked by a native speaker.
The language changes the app around your budget, not what you typed into it: line descriptions, notes and job details stay as written. PDFs and spreadsheets have their own language setting in the Export dialog.
Not what you were looking for? Write to hello@actualsbudget.com.