DIctionary Mojave

You will need to find a website where you can download a suitable dictionary file, I could only find this.

http://clasquin-johnson.co.za/michel/mac-os-x-dictionaries/east-european-languag es/bulgarian.html

Once the files are downloaded move them to your Users/<Home>/Library/Dictionaries folder. Better still open the Dictionary app, click on File in the menubar and then click on Open Dictionary Folder, now drag the files into the window, see below.

Now close the Dictionary app, then reopen it, select Preferences from File in the menubar, the new Dictionaries should be at the bottom of the list, select them to make them active, and move them up the list in order of preference.

Please note if you are using Dark Mode it will be difficult to read the results.

i100 wrote:

noticed that there is no more Bulgarian-English-Bulgarian dictionary on the Dictionary app.

Apple has never provided such a dictionary, so if you had one, you got it from the internet. Try to remember where. Another place to look is

https://dictionaries.io/