Multiple languages for SLAs and the SLA UI can be added via the CPack
variables CPACK_DMG_SLA_DIR and CPACK_DMG_SLA_LANGUAGES. For each
language defined in the languages variable, CPack will search for
<language>.menu.txt and <language>.license.txt in CPACK_DMG_SLA_DIR.
If the sla directory variable is not defined, the old behaviour using
CPACK_RESOURCE_FILE_LICENSE is retained.
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