runtime(lang): Revise Makefile (#13589)

* Revise runtime/lang/Makefile
* Use predefined variables (`$@`, `$<`) instead of the actual file names.
  (Adding new rules should become easier.)
* Adjust spacing.
* Regenerate converted menu files

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/lang/menu_sr_rs.iso_8859-5.vim b/runtime/lang/menu_sr_rs.iso_8859-5.vim
index 870506f..f0b5b37 100644
--- a/runtime/lang/menu_sr_rs.iso_8859-5.vim
+++ b/runtime/lang/menu_sr_rs.iso_8859-5.vim
@@ -12,7 +12,7 @@
 let s:keepcpo= &cpo
 set cpo&vim
 
-scriptencoding iso8859-5
+scriptencoding iso-8859-5
 
 " Help menu
 menutrans &Help		      ¿ÞÜÞ&û