I believe this has been resolved in Odoo version 13 & 14, just activate Multi-Currencies.  In  Opening  journal  entry enter  2nd  currency  amount  in  "Amount  in  Currency" choose  2nd  currency . The  conversion will be  done automatically in  company  default  currency  (you  should  have  already  entered  currency  rates).  Also you  can  edit  the  converted  amount  as  well.   

This is a very important issue, and seems there is a bug somewhere in the system.

If I try to book an EUR Journal Entry in an USD base environment, it seems the system does not calculate the amount with the given Exchange rate.

I also tried to create e new Journal(EUR) and I put the EUR as the secondary currency in the involved accounts.

But somewhere in the system this calculation is made: when I try to print an involved account (Reporting-Legal Reports-Accounting reports-General Ledger) and I search it, in the list of result of search appears the account (Travel Expenses for example) with the field "adjusted balalce". In this field appears the correct USD amount, corrsponding to the EUR amount of the original transaction. But when I finally print the report, this information is lost. How is possible? this is not a complicate issue, rather a usual kind of booking.

Hi,

Unfortunately there isn't any special module available to calculate currency value from debit/credit on account move line. However, as a work around you can use journal voucher to post entries with foreign currency. Remember: You need to force account and journal to post in foreign currency amount. 

Hope this would help. 

You right because in journal entry form you doesn't have onchange on currency field. But if you try to generate the journal entry by invoice you will have the result you're expecting.