Add language validation & refactor config initialization

src/main/java/at/pcgamingfreaks/mkvaudiosubtitlechanger/util/LanguageValidatorUtil.java

@@ -0,0 +1,44 @@
+package at.pcgamingfreaks.mkvaudiosubtitlechanger.util;
+
+import at.pcgamingfreaks.mkvaudiosubtitlechanger.config.ConfigErrors;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+public class LanguageValidatorUtil {
+    private static Set<String> ISO3_LANGUAGES;
+
+    static {
+        try {
+            ISO3_LANGUAGES = load("language-codes");
+        } catch (IOException ignored) {}
+    }
+
+    private static Set<String> load(String path) throws IOException{
+        if (new File(path).isFile()) {
+            return Files.lines(Path.of(path)).collect(Collectors.toSet());
+        } else {
+            try(BufferedReader bf = new BufferedReader(new InputStreamReader(
+                    Objects.requireNonNull(LanguageValidatorUtil.class.getClassLoader().getResourceAsStream(path))))) {
+                return bf.lines().collect(Collectors.toSet());
+            }
+        }
+    }
+
+    public static boolean isLanguageValid(String language) {
+        return ISO3_LANGUAGES.contains(language);
+    }
+
+    public static void isLanguageValid(String language, ConfigErrors errors) {
+        if (!isLanguageValid(language)) {
+            errors.add(String.format("%s is not a valid language", language));
+        }
+    }
+}