Yes, it is possible to translate your Flutter app for free using machine translation tools such as Google Translate. However, the quality of the translation may not be as good as when people translate it, and machine translation may not translate placeholders and ICU messages well.
A better option is to use a translation management platform like Localizely. Localizely offers a free plan that may be enough for mobile apps that don't have a large number of string keys. Translation management platforms like Localizely offer more than just machine translation.
They also offer collaboration during the translation process, with admins and translators working together. They integrate with GitHub, GitLab, and other services, allowing for easy synchronization of changes. They offer review of translations, glossaries, and translation memory that help ensure quality translations and reduce expenses by not translating terms that have already been translated.