SELECT 
  pfv.product_id, 
  pfvd.variant 
FROM 
  cscart_product_features_values AS pfv 
  LEFT JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfvd.variant_id = pfv.variant_id 
  AND pfvd.lang_code = 'de' 
WHERE 
  pfv.feature_id = '5'

Query time 0.04861

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4500.81"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "fpl"
          ],
          "key": "fl",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 9706,
          "rows_produced_per_join": 9706,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "133.11",
            "eval_cost": "970.60",
            "prefix_cost": "1103.71",
            "data_read_per_join": "7M"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "dbggbern.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 9706,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2426.50",
            "eval_cost": "970.60",
            "prefix_cost": "4500.81",
            "data_read_per_join": "28M"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}