SELECT 
  cscart_languages.* 
FROM 
  cscart_languages 
  LEFT JOIN cscart_storefronts_languages AS storefronts_languages ON storefronts_languages.language_id = cscart_languages.lang_id 
WHERE 
  1 
  AND (
    storefronts_languages.storefront_id = 1 
    OR storefronts_languages.storefront_id IS NULL
  )

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_languages",
      "access_type": "ALL",
      "rows": 2,
      "filtered": 100
    },
    "block-nl-join": {
      "table": {
        "table_name": "storefronts_languages",
        "access_type": "index",
        "key": "PRIMARY",
        "key_length": "8",
        "used_key_parts": ["storefront_id", "language_id"],
        "rows": 1,
        "filtered": 100,
        "using_index": true
      },
      "buffer_type": "flat",
      "buffer_size": "2Kb",
      "join_type": "BNL",
      "attached_condition": "trigcond(storefronts_languages.storefront_id = 1 or storefronts_languages.storefront_id is null) and trigcond(storefronts_languages.language_id = cscart_languages.lang_id)"
    }
  }
}