The answer depends on how your content is hosted at Transifex.
Localized content is hosted at different TX projects. In this case, API is recommended.
Specifically, you can assign tags to your projects (through the project’s settings page or API) and then get the details for each project via API - the project’s tags will be included in the response. This information will allow you to group your projects based on the assigned tags. More information about this can be found in our documentation guide here.
Localized content is hosted on a single project. In this case, you can group the resources instead via Web Interface.
Specifically, you can assign categories to your resources as described in our documentation guide here and use the available filters to get only the resources that belong to a specific category.