I suppose you want to achieve something like this

Let me give you 2 example of unicodes for country flags:
for ROMANIA ---> \uD83C\uDDF7\uD83C\uDDF4
for AMERICA ---> \uD83C\uDDFA\uD83C\uDDF8
You can get this and other country flags unicodes from this site Emoji Unicodes
Once you enter the site, you will see a table with a lot of emoji. Select the tab with FLAGS from that table (is easy to find it) then will appear all the country flags. You need to select one flag from the list, any flag you want... but only ONE. After that will appear a text code in the message box...that is not important. Important is that you have to look in the right of the site where will appear flag and country name of your selected flag. CLICK on that, and on the page that will open you need to find the TABLE named Emoji Character Encoding Data. Scroll until the last part of table where sais: C/C++/Java Src .. there you will find the correct unicode flag. Attention, always select the unicode that is long like that, some times if you are not carefull you can select a simple unicode, not long like that. So, keep that in mind.
Indications image 1

Indication image 2

In the end i will post a sample code from an Android app of mine that will work on java the same way.
ArrayList<String> listLanguages = new ArrayList<>();
listLanguages.add("\uD83C\uDDFA\uD83C\uDDF8 " + getString(R.string.English));
listLanguages.add("\uD83C\uDDF7\uD83C\uDDF4 " + getString(R.string.Romanian));
Another simple custom example:
String flagCountryName = "\uD83C\uDDEF\uD83C\uDDF2 Jamaica";
You can use this variable where you need it. This will show you the flag of Jamaica in front of the text.
This is all, if you did not understand something just ask.