Using COLLATE with UNION
Based on the collation names I assume that you are using Microsoft SQL Server. 

COLLATE can be used at the database level or the column level.   Since you are trying to UNION two tables, using the column collation on the needed columns will resolve your query.

Here is a sample bit of code to help you:

use testdb
GO
CREATE TABLE dbo.Serbian  (Name VARCHAR(20) COLLATE Serbian_Latin_100_CI_AS);
CREATE TABLE dbo.Croatian (Name VARCHAR(20) COLLATE  Croatian_CI_AS);
GO
INSERT INTO dbo.Serbian VALUES ('serbian');
INSERT INTO dbo.Croatian VALUES ('croation');
GO

-- Collate to a particular named collation
SELECT Name COLLATE Serbian_Latin_100_CI_AS as CollatedNameSerbian from dbo.Serbian 
UNION ALL
SELECT Name COLLATE Serbian_Latin_100_CI_AS from dbo.Croatian 
GO
-- Collate to the database default collation
SELECT Name  COLLATE database_default as CollatedNameDBDefault from dbo.Serbian 
UNION ALL
SELECT Name COLLATE database_default from dbo.Croatian 
GO

DROP TABLE dbo.Serbian;
DROP TABLE dbo.Croatian;
GO


Of course, if you have several columns with conflicting collations you will need to define their collations as well.