Here what the Help says:
Resource bundle is a set of properties files that have same base name with different language-specific suffixes. A resource bundle contains at least two properties files with similar base name, for example file_en.propertiesand file_de.properties. IntelliJ IDEA recognizes properties files, and if two or more properties files with the names that differ only in suffix, are encountered, joins them into a resource bundle. The new node Resource Bundle '(base name)'appears in the Project Tool Window: I sure don't see any bundling of the matching properties files in my IDE, version 11.1.1 build IC.117.117. And I don't see any way to affect this in project or IDE settings.