SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_languages AS languages ON storefronts.storefront_id = languages.storefront_id 
WHERE 
  1 = 1 
  AND (
    languages.language_id IN ('3')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.008506245,
    "filesort": {
      "sort_key": "storefronts.is_default desc",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "languages",
              "access_type": "index",
              "possible_keys": ["PRIMARY", "idx_storefront_id"],
              "key": "idx_storefront_id",
              "key_length": "4",
              "used_key_parts": ["storefront_id"],
              "loops": 1,
              "rows": 1,
              "cost": 0.006792605,
              "filtered": 100,
              "attached_condition": "languages.language_id = '3'",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "storefronts",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "4",
              "used_key_parts": ["storefront_id"],
              "ref": ["u985510652_ecartify.languages.storefront_id"],
              "loops": 1,
              "rows": 1,
              "cost": 0.00171364,
              "filtered": 100,
              "attached_condition": "storefronts.storefront_id = languages.storefront_id"
            }
          }
        ]
      }
    }
  }
}