Updates to the translations get integrated whenever a new version of 0ad gets released.
Once we start the release process, we'll publish multiple release candidates, translators like you can use to check if the translations look fine in the game and give you the chance to polish the translations.
The start of the release process will be announced via a notification on Transifex, as well as in the "Announcements / News" subforum here.
If you're eager to try out the translation before, you can either compile the current development state of the game yourself or, if you're using Windows, use the nightly build: https://gitea.wildfiregames.com/0ad/0ad/wiki/NightlyBuild