SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '2998', '4796', '4792', '4766', '4763', 
    '4779', '4784', '5163', '5164', '5179', 
    '5180', '4754', '1882', '3096', '3095', 
    '3097', '3098', '4981', '3099', '3100', 
    '3101', '3102', '3104', '5176', '5029', 
    '4974', '4983', '3085', '5063', '4915', 
    '3087', '3686', '3088', '3089', '3090', 
    '3091', '3092', '3093', '3094', '3084', 
    '5182', '1273', '1404', '4904', '4894', 
    '1856', '1870', '4905', 'Y'
  ) 
  AND lang_code = 'ru'

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variant_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "rows": 49,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('2998','4796','4792','4766','4763','4779','4784','5163','5164','5179','5180','4754','1882','3096','3095','3097','3098','4981','3099','3100','3101','3102','3104','5176','5029','4974','4983','3085','5063','4915','3087','3686','3088','3089','3090','3091','3092','3093','3094','3084','5182','1273','1404','4904','4894','1856','1870','4905','Y') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}