There needs to be a way to report all language codes that have had e.g. fewer than 25 edits in the past 3 months (bot edits excluded).

Rationale: translators in a language are usually active across multiple projects or message groups; by easily identifying less active languages, it's possible to reach out to (potential) translators for that language and help all projects.

The easiest way would probably be adding the feature to scripts/languagestats.php.
Another possibility could be to add an option to the special pages: LanguageStats gives only completion stats for a single language and MessageGroupStats the same for any single group; both could maybe have a column with the number of recent edits so that everyone can find groups or languages in need of improvement in any language or group.