differentiate depending on used tex engine

agse-thesis.cls

 \NeedsTeXFormat{LaTeX2e}
 \ProvidesClass{agse-thesis}[2021/03/31 v0.2 AGSE Thesis]
 
+\RequirePackage{iftex}
 %%% Read options
 % Language: Default is German
 \newcommand{\lang}{ngerman}
@@ -45,8 +46,12 @@
     \RequirePackage{paratype}
     \renewcommand*\familydefault{\sfdefault}
 }{}
-%\RequirePackage[T1]{fontenc}
-\RequirePackage{fontspec}
+% load font for special characters depending on engine
+\ifLuaTeX
+  \RequirePackage{fontspec}
+\else
+  \RequirePackage[T1]{fontenc}
+\fi
 
 % Allow unicode in input files
 \RequirePackage[utf8]{inputenc}