Decision:
Set default_language and language_override to the same literal value (in your case, "ru").
How did I get here...
I ran into the same problem, also on Mongo 2.6.1.
In my case, I created an index with language_override , pointing to a language field where there were already documents with unsupported values (for example, "ar" is Arabic).
This is how I created the index:
db.users.ensureIndex({ "displayName": "text", "about": "text", "email": "text" }, { "name": "users_full_text", "default_language": "en", "language_override": "language" });
I was hoping it would return to default_language when the language_override value is not supported, but apparently not. Here is what Mongo says:
{ "createdCollectionAutomatically" : false, "numIndexesBefore" : 3, "ok" : 0, "errmsg" : "language override unsupported: ar", "code" : 17262 }
OK, great, so the index was not created, but I would have to create it without language_image, right? Wrong - mongo gives me the same error even if I no longer have the specified language_override.
The failed attempt to create the index seems to have left behind some broken version of the index that does not appear anywhere, so I cannot drop it (it does not appear in db.users.getIndexes() and omits it by the name doesn work).
In the end, I managed to fix the index by setting language_override to the literal value 'en', for example:
db.users.ensureIndex({ "displayName": "text", "about": "text", "email": "text" }, { "name": "users_full_text", "default_language": "en", "language_override": "en" });
... to which Mongo replies:
{ "createdCollectionAutomatically" : false, "numIndexesBefore" : 3, "numIndexesAfter" : 4, "ok" : 1 }
Hurray.