latex and xetex support

examplelttr.tex

@@ -1,7 +1,9 @@
 \documentclass[unibrief]{scrlttr2}
 
 \usepackage[ngerman]{babel}  % change language here
-\usepackage[utf8]{inputenc}  %  whatever input encoding you use
+\ifpdftex
+   \usepackage[utf8]{inputenc}  %  whatever input encoding you use
+\fi
 \usepackage[math]{blindtext} % remove this line
                       % (unless you need \blindtext)  
 \begin{document}

unibrief.lco

@@ -7,19 +7,39 @@
         fontsize=10pt,
         foldmarks=MPvb,
         backaddress=plain,
-%        fromlogo,
-%        fromalign=right,
         pagenumber=footright,
         refline=nodate
 }
 
-
 % font stuff
-\RequirePackage[stretch=10]{microtype}
-\RequirePackage{libertine}
-\RequirePackage[T1]{fontenc}
-\RequirePackage[libertine]{newtxmath}
+\RequirePackage{iftex}
+\ifpdftex
+    \RequirePackage{libertine}
+    \RequirePackage[libertine]{newtxmath}
+    \RequirePackage[T1]{fontenc}
+\else\ifxetex
+    \RequirePackage{fontspec}
+    \RequirePackage{unicode-math}
+    \setmainfont[
+        Extension=.otf,
+        UprightFont={*-Regular},
+        BoldFont={*-Bold},
+        ItalicFont={*-Italic},
+        BoldItalicFont={*-BoldItalic}]{LibertinusSerif}
+    \setmathfont[Scale=MatchUppercase]{LibertinusMath-Regular.otf}
+\else\ifluatex
+    \RequirePackage{fontspec}
+    \RequirePackage{unicode-math}
+    \setmainfont[
+        Extension=.otf,
+        UprightFont={*-Regular},
+        BoldFont={*-Bold},
+        ItalicFont={*-Italic},
+        BoldItalicFont={*-BoldItalic}]{LibertinusSerif}
+   \setmathfont[Scale=MatchUppercase]{LibertinusMath-Regular.otf}
+\fi\fi\fi
 
+\RequirePackage[stretch=10]{microtype}
 \RequirePackage{lastpage}
 \RequirePackage{graphicx}