As described in Externalized Strings, all string values in UIM files are externalized to .properties files.
If MyPage.uim is the UIM file, then MyPage.properties is the corresponding properties file. To add localized properties files, please see Locales.
The strings are stored in a properties file in the same folder as the page or view file. This file must have the same name as the page or view file but with the extension .properties. For example, if the page is stored in a file called MyPage.uim, the strings will be stored in the file MyPage.properties in the same folder. Similarly, views will see the .vim extension changed to .properties.
While UIM documents in the highest priority component override those in all other components, properties files in different components are merged together. Individual properties override those with the same property name defined in lower priority components. Also, when a UIM page includes a UIM view (a .vim file), all of the properties defined for both the page and the view are merged and the properties for the page override those defined for the view where they share the same property name. These two merging steps happen separately with the component order applied first for each properties file and the page-view order applied on the resulting properties. A property defined for a page will override a property of the same name defined for a view, even if the property for the view was defined in a higher priority component.