Support for international letters like øæå. (#13316)

Co-authored-by: Sverre Kristian Valskrå <Sverre.Valskra@skatteetaten.no>

Author: sverrevalskraa

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractKotlinCodegen.java

@@ -35,6 +35,7 @@
 import java.io.IOException;
 import java.util.*;
 import java.util.function.Function;
+import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -778,7 +779,7 @@ private String sanitizeKotlinSpecificNames(final String name) {
         }
 
         // Fallback, replace unknowns with underscore.
-        word = word.replaceAll("\\W+", "_");
+        word = Pattern.compile("\\W+", Pattern.UNICODE_CHARACTER_CLASS).matcher(word).replaceAll("_");
         if (word.matches("\\d.*")) {
             word = "_" + word;
         }