First of all, you have to replace the default messages to the custom messages. And to do this you have to create and specify the hidden page items in the SUCCESS and Error message areas. You can follow these steps:

You can create a table to store your messages in the Bosnian language. For example:

Create Table bosnian_messages (
       msg_id integer,
       bosnian_msg nvarchar2(1000),
       message_type varchar2(100))
/

Insert the data into the above table as follows:

Insert into bosnian_messages (msg_id, bosnian_msg, message_type)
   values (1, 'Datoteka je uspješno preuzeta.', 'FILE_DOWNLOAD_SUCCESS');

Now to show the messages using the alert, you can use Oracle Apex shortcuts in shared components, or you can assign the message on page load to a hidden item.

For example, create a hidden item (P2_SUCCESS_MSG) on the page you are downloading the file and set the default value as the following SQL query:

Select bosnian_msg from bosnian_messages
   where msg_id = 1 and message_type = 'FILE_DOWNLOAD_SUCCESS';

Now on page load, the item P2_SUCCESS_MSG will be populated with the message. You can then show it using the alert as follows:

apex.message.alert(apex.item('P2_SUCCESS_MSG').getValue());

Please let me know if any questions.