SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1230', '5901', '1255', '5903', '5913', 
    '5907', '1425', '1280', '1254', '5909', 
    '5912', '1199', '5914', '1241', '5915', 
    '5916', '5940'
  ) 
  AND lang_code = 'ru'

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 17,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1230','5901','1255','5903','5913','5907','1425','1280','1254','5909','5912','1199','5914','1241','5915','5916','5940') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}