SELECT 
  variant_descriptions.variant 
FROM 
  cscart_product_features_values AS v 
  LEFT JOIN cscart_product_feature_variant_descriptions AS variant_descriptions ON variant_descriptions.variant_id = v.variant_id 
  AND v.lang_code = variant_descriptions.lang_code 
WHERE 
  v.product_id = 142 
  AND v.feature_id = 124 
  AND variant_descriptions.lang_code = 'de'

Query time 0.00022

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.002862638,
    "nested_loop": [
      {
        "table": {
          "table_name": "v",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "variant_id",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["feature_id", "product_id"],
          "ref": ["const", "const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.000928812,
          "filtered": 5.36193037,
          "attached_condition": "v.lang_code = 'de'",
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "variant_descriptions",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "ref": ["cscart.v.variant_id", "const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.001933826,
          "filtered": 100,
          "index_condition": "variant_descriptions.lang_code = v.lang_code"
        }
      }
    ]
  }
}