When we tested the polylang with the WP Travel plugin, we found the issue. Again, when we tested the polylang plugin deactivating the wp travel plugin in default twenty seventeen theme for confirmation, the issue persists.
So it is actually the issue of polylang so please contact with the respective forum of polylang for the issue.
Further, if you have any query related to the wp travel plugin then let us know.
this what it wrote the polylang plugin author
” By default, only *public* post types are shown in the options. I guess that when a plugin author decides that a post type is not public, then he must have good reasons and that’s enough for me to decide not to reveal such post type in Polylang options. The same for taxonomies.
You can always control which post types and taxonomies are translatable by using a wpml-config.xml file or the filter pll_get_post_types (including dor non-public post types and taxonomies). ”