SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'ru' 
WHERE 
  pfv.variant_id IN (
    3090, 3087, 3100, 3099, 3089, 3101, 3093, 
    3092, 3104, 3686, 3096, 3102, 3094, 
    3103, 3097, 3085, 3084, 3095
  )

Query time 0.00070

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 18,
          "filtered": 100,
          "index_condition": "pfv.variant_id in (3090,3087,3100,3099,3089,3101,3093,3092,3104,3686,3096,3102,3094,3103,3097,3085,3084,3095)"
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "ref": ["lillyshop_lillyshop.pfv.variant_id", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "pfvd.lang_code = 'ru'"
        }
      }
    ]
  }
}