Add Okanagan language

Requested at
https://translatewiki.net/wiki/Thread:Support/Colville-Okanagan_Salish

Autonym spelling according to
http://www.interiorsalish.com/images/ns_601_lxcin_1_book_3rd_Ed_Vers_2.pdf

data/langdb.yaml

@@ -391,6 +391,7 @@ languages:
   ny: [Latn, [AF], Chi-Chewa]
   nys: [Latn, [PA], Nyungar]
   oc: [Latn, [EU], occitan]
+  oka: [Latn, [AM], n̓səl̓xcin̓]
   olo: [Latn, [AS, EU], Livvinkarjala]
   om: [Latn, [AF], Oromoo]
   ood: [Latn, [AM], "ʼOʼodham ha-ñeʼokĭ"]

language-data.json

@@ -2518,6 +2518,13 @@
             ],
             "occitan"
         ],
+        "oka": [
+            "Latn",
+            [
+                "AM"
+            ],
+            "n̓səl̓xcin̓"
+        ],
         "olo": [
             "Latn",
             [