Add language detection via "auto" language

Fixes #103.

changelog.xml

@@ -16,6 +16,9 @@
       <action type="update">
         Refactor CLI into `ltex-cli`; `--input-documents` and `--setings-file` are deprecated and will be removed in a future release
       </action>
+      <action type="add" issue="#103">
+        Add support for automatic language detection via language short code `auto`; language variants like `en-US` are not detected, only generic languages like `en`; this will result in spelling errors not being reported
+      </action>
       <action type="add">
         Provide ID of LanguageTool rule via diagnostics code, not as part of diagnostics message
       </action>

src/main/kotlin/org/bsplines/ltexls/server/DocumentChecker.kt

@@ -21,6 +21,8 @@ import org.bsplines.ltexls.tools.I18n
 import org.bsplines.ltexls.tools.Logging
 import org.bsplines.ltexls.tools.Tools
 import org.eclipse.lsp4j.Range
+import org.languagetool.Language
+import org.languagetool.language.LanguageIdentifier
 import org.languagetool.markup.AnnotatedText
 import org.languagetool.markup.TextPart
 import java.time.Duration
@@ -33,6 +35,8 @@ class DocumentChecker(
   var lastCheckedDocument: LtexTextDocumentItem? = null
     private set
 
+  private val languageIdentifier = LanguageIdentifier()
+
   private fun fragmentizeDocument(
     document: LtexTextDocumentItem,
     range: Range?,
@@ -86,8 +90,20 @@ class DocumentChecker(
     rangeOffset: Int,
   ): List<LanguageToolRuleMatch> {
     val codeFragment: CodeFragment = annotatedTextFragment.codeFragment
-    val settings: Settings = codeFragment.settings
+    var settings: Settings = codeFragment.settings
+
+    if (settings.languageShortCode == "auto") {
+      val cleanText: String = this.languageIdentifier.cleanAndShortenText(
+        annotatedTextFragment.annotatedText.plainText,
+      )
+      val language: Language? = this.languageIdentifier.detectLanguage(cleanText)
+      settings = settings.copy(
+        _languageShortCode = language?.shortCodeWithCountryAndVariant ?: "en-US",
+      )
+    }
+
     this.settingsManager.settings = settings
+
     val languageToolInterface: LanguageToolInterface =
         this.settingsManager.languageToolInterface ?: run {
           Logging.logger.warning(I18n.format(

src/test/kotlin/org/bsplines/ltexls/server/DocumentCheckerTest.kt

@@ -118,15 +118,28 @@ class DocumentCheckerTest {
       """
       This is an **test.**
 
-      [comment]: <> "LTeX: language=de-DE"
+      <!-- LTeX: language=de-DE -->
 
       Dies ist eine **Test**.
 
       """.trimIndent()
     )
-    val checkingResult: Pair<List<LanguageToolRuleMatch>, List<AnnotatedTextFragment>> =
-      checkDocument(document)
-    assertMatches(checkingResult.first, 8, 10, 69, 80)
+    assertMatches(checkDocument(document).first, 8, 10, 62, 73)
+  }
+
+  @Test
+  fun testLanguageDetection() {
+    val document: LtexTextDocumentItem = createDocument("markdown",
+      """
+      This is an **test.**
+
+      <!-- LTeX: language=auto -->
+
+      Dies ist eine **Test**.
+
+      """.trimIndent()
+    )
+    assertMatches(checkDocument(document).first, 8, 10, 61, 72)
   }
 
   @Test