Steps to reproduce

1.  Add to the travel site's i18n for example the  szl locale (for "Silesian", https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry). Just the lang locale, not any country. 
2. In page editor, edit page name (via page properties)  of /travel/book-tour/meal. In the language switcher, choose Silesian, and rename the page to "Krutipysk" .
   
3. Retrieve the page's parent via REST, will pose two problems:
   Endpoint config:

class: info.magnolia.rest.delivery.jcr.v2.JcrDeliveryEndpointDefinition
workspace: website
bypassWorkspaceAcls: true
rootPath: /travel/book-tour/
nodeTypes:
 - mgnl:page

• First problem, try getting the szl variant itself:
  http://localhost:8080/magnoliaAuthor/.rest/delivery/ep/?lang=szl

Result returned - the (three-letter?) locale "is not allowed": 

{
    "error": {
        "code": "badRequest",
        "message": "Language parameter: szl is not allowed"
    }
}

Expected (part) is:

{
"@name": "meal",
"@path": "/travel/book-tour/meal",
"@id": "9b8ea1b2-a93f-4015-a822-84d730ca982a",
"@nodeType": "mgnl:page",
"hideInNav": "true",
"title": "Krutipysk",
"@nodes":[]
}

• Second problem, try getting a different locale, e.g. de:
  http://localhost:8080/magnoliaAuthor/.rest/delivery/ep/?lang=de

Result returned - notice that it includes also the szl variant: 

{
"@name": "meal",
"@path": "/travel/book-tour/meal",
"@id": "9b8ea1b2-a93f-4015-a822-84d730ca982a",
"@nodeType": "mgnl:page",
"hideInNav": "true",
"title": "Menü",
"title_szl": "Krutipysk",
"@nodes":[]
}

Expected (part) is:

{
"@name": "meal",
"@path": "/travel/book-tour/meal",
"@id": "9b8ea1b2-a93f-4015-a822-84d730ca982a",
"@nodeType": "mgnl:page",
"hideInNav": "true",
"title": "Menü",
"@nodes":[]
}

Possible impact of the issue

May affect for example the speakers of Chinese who would like to configure in a specific variant rather then the zh macrolanguage locale:
cmn - "... Mandarin is often placed first in lists of languages by number of native speakers (with nearly a billion). Mandarin is by far the largest of the seven or ten Chinese dialect groups, spoken by 70 percent of all Chinese speakers over a large geographical area ..."

Note: The IANA subtag registry lists:

• 8326 three-letter lang subtags
• 190 two-letter lang subags