A language does not need to be 100% complete to be included in a release. Generally, we'll include whatever translations are present - AtoM uses what is called "culture fallback" meaning that whenever there is not a translated string available, AtoM will "fall back" to displaying the original English source string instead, rather than nothing. This is in place for both user interface translations provided by the community, as well as translations of content added to records by users.
With our move to Weblate, we have tried to introduce a new review process to help improve the quality of our translations long-term. For strings to be added to a release, they need to have a reviewer mark them as approved. We have more information on this, and steps to request becoming a reviewer, in our wiki documentation, here:
However, this is a fairly big change in our translations process, and since the translations are entirely volunteer-driven, often by just one or two people per language, not every translation project will have a reviewer yet! As such, for the 2.5 release (and possibly for the 2.6 release as well), we created some local scripts that, once our team has done some spot checking to ensure there's no intentional spam etc in the provided translations, will automatically approve all strings before merging them into the release. As more of our volunteers move over to the new Weblate translation platform and contact us to become reviewers, we will eventually phase out this automatic addition, so that all strings are being reviewed and approved before being added to a public release.
Right now, in Weblate, we still only have one open project - 2.5. This is while we wait for feedback from the community on the 2.5.0 release and determine when we think 2.6 will be ready, to decide if we need to prepare a 2.5.1 bug fix release. You can add translations to the 2.5 project in Weblate now. If we do create a 2.5.1 release, they will be included. If we don't, then we will lock 2.5 in Weblate, copy all the strings to a new 2.6 project, update the weblate resources to include any new strings from the 2.6 release, and then add any translations in the new project to the 2.6 release when it's ready.
Either way, we'll send out an announcement a couple weeks before we lock the translations in preparation for the next release, so our community has some time to wrap up ongoing work and get ready for the release.