Legalization

For the States that are Parties to the Hague Convention, it shall be executed the so-called simplified legalization, which consists of placing an Apostille stamp, as a result of which the document acquires legal force in all the countries that have acceded to the Convention.
You may find detailed information about the States Parties to the Hague Convention here. 

For the countries with which the Republic of Bulgaria has signed and still in force bilateral treaties on legal assistance, which include texts for the exemption of the documents from legalization, it is enough the documents, issued by the foreign country, to bear a stamp of the institution authorized in the treaty. The document thus issued is recognized by the Bulgarian institutions and needs only translation into Bulgarian and authentication of the translator's signature. 

Some documents for civil status issued by and intended to serve before the countries that have signed the Convention on the issue of multilingual extracts from civil status records, which is signed on 08.09.1976 in Vienna (in force for the Republic of Bulgaria since 18.12.2013) also may be exempted from legalization. 

The validity of documents from countries that are not Parties to the Hague Convention and with which Bulgaria hasn’t got concluded treaties on legal assistance, shall be certified according to the full legalization procedure.