As described here
Wikidata is currently 7 GB and growing fast. Some users may only need a small portion of Wikidata. For example, German Wikipedia only needs 10% of Wikidata to function.
Schnark wrote a perl script to selectively filter Wikidata to German-related pages only. As perl is not present on Windows by default, XOWA will instead incorporate the same functionality directly into its import process.
This feature will be implemented once I get requests for it, or once Wikidata grows much larger (for example, >10 GB)