Languages which a shop use.

It is used to select dictionary, currency, address and so on.

There must be at least one relation for a shop.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
lang	TEXT		Language key (Terminological code of ISO 639-2).
index	INTEGER	NOT NULL	Ordering of languages. 0-index record is regarded as default language.

CREATE TABLE shop_lang_map(
shop     TEXT,
lang     TEXT,
index    INTEGER  NOT NULL,
PRIMARY KEY(shop, lang)
);