Best way to convert text files between character sets?

Question

What is the fastest, easiest tool or method to convert text files between character sets?

Specifically, I need to convert from UTF-8 to ISO-8859-15 and vice versa.

Answer 1

to write properties file (Java) normally I use this in linux (mint and ubuntu distributions):

$ native2ascii filename.properties

For example:

$ cat test.properties 
first=Execução número um
second=Execução número dois

$ native2ascii test.properties 
first=Execu\u00e7\u00e3o n\u00famero um
second=Execu\u00e7\u00e3o n\u00famero dois

PS: I writed Execution number one/two in portugues to force special characters.

In my case, in first execution I received this message:

$ native2ascii teste.txt 
The program 'native2ascii' can be found in the following packages:
 * gcj-5-jdk
 * openjdk-8-jdk-headless
 * gcj-4.8-jdk
 * gcj-4.9-jdk
Try: sudo apt install 

When I installed the first option (gcj-5-jdk) the problem was finished.

I hope this help someone.