languagetool-org/languagetool · error · RuntimeException

Not supported: " + fromLang + " -> " + toLang

Error message

Not supported: " + fromLang + " -> " + toLang

What it means

BeoLingusTranslator.translate only supports German->English and English->German lookups against its two in-memory maps. Any other language pair passed in fromLang/toLang throws a RuntimeException naming the unsupported direction.

Source

Thrown at languagetool-language-modules/en/src/main/java/org/languagetool/rules/en/translation/BeoLingusTranslator.java:183

        mergedList.add(item);
        merging = true;
      } else {
        mergedList.add(item);
      }
      mergeListPos++;
    }
    return mergedList;
  }

  @Override
  public List<TranslationEntry> translate(String term, String fromLang, String toLang) {
    Map<String, List<TranslationEntry>> map;
    if (fromLang.equals("de") && toLang.equals("en")) {
      map = this.de2en;
    } else if (fromLang.equals("en") && toLang.equals("de")) {
      map = this.en2de;
    } else {
      throw new RuntimeException("Not supported: " + fromLang + " -> " + toLang);
    }
    List<TranslationEntry> entries = map.get(term.toLowerCase());
    Set<TranslationEntry> entriesSet = new HashSet<>();
    if (entries != null) {
      entriesSet.addAll(entries);
    }
    List<TranslationEntry> translationsForBaseforms = getTranslationsForBaseforms(term, map);
    for (TranslationEntry trans : translationsForBaseforms) {
      if (entries != null) {
        Optional<TranslationEntry> first = entries.stream().filter(k -> k.getL1().equals(trans.getL1())).findFirst();
        if (first.isPresent() && first.get().getL1().equals(trans.getL1())) {
          // skip duplicates
        } else {
          entriesSet.add(trans);
        }
      } else {
        entriesSet.add(trans);
      }

View on GitHub (pinned to 2e990059ce)

Solutions

1. Only call translate with fromLang/toLang equal to "de"->"en" or "en"->"de".
2. Validate the language pair before calling and route other pairs to a different translation component.
3. If another direction is required, extend the translator to populate and consult an additional map.

Example fix

// before
translator.translate(term, "fr", "en");
// after
if (fromLang.equals("de") && toLang.equals("en")) {
  translator.translate(term, "de", "en");
}

Set<List<String>> supported = Set.of(List.of("de","en"), List.of("en","de"));
if (!supported.contains(List.of(fromLang, toLang))) {
  throw new UnsupportedOperationException("Pair not supported: " + fromLang + "->" + toLang);
}

try {
  translator.translate(term, fromLang, toLang);
} catch (RuntimeException e) {
  if (e.getMessage().startsWith("Not supported:")) { /* fall back to another translator */ }
}

When it happens

Trigger: Calling translate(term, "fr", "en") or any language pair other than ("de","en") and ("en","de") on a BeoLingusTranslator instance.

Common situations: Reusing the translator in code that handles multiple language pairs; passing language codes in the wrong case or wrong variable; assuming the class supports the file's other translation directions (en2de is loaded but noted as not populated).

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• <regexp> rules currently cannot be used together with <antip
• Could not tag '" + term + "'
• Not implemented
• Format error in file ${path}, line: ${line}
• No IssueType found for name '" + name + "'. Valid values: "

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/12ec2c63e8c9da46. Report an issue: GitHub.