Please see https://github.com/silverstripe/silverstripe-forum/tree/master/docs/en
Translations of the natural language strings are managed through a third party translation interface, transifex.com. Newly added strings will be periodically uploaded there for translation, and any new translations will be merged back to the project source code.
Please use https://www.transifex.com/projects/p/silverstripe-forum/ to contribute translations, rather than sending pull requests with YAML files.
See the "i18n" topic on doc.silverstripe.org for more details.