Version ADempiere 3.1.6 (and older and some higher version of ADempiere ) dont have Serbian Language into Login form in select language. This is very simple to add new Language or change existing, or set default one of the prefer language. Theres path where is located java files which should be changed or added -- ../looks/src/org/compiere /plaf/PlafRes_sr.java ../looks/src/org/compiere/util/IniRes_sr.java ../looks/src/org/compiere/util/Language.java.
And for full transformation, should be add translation from : http://sourceforge.net/projects/lsr/ -- with adding Serbia_RS and set as main language.
New java file should be add :PlafRes_sr.java
private static final String AD_Language_sr_RS = "sr_RS" -- should be add into Language.java
AD_Language should be add into Language.java
compile all java files