It's harder then it first seems. The root problem is that for text like "hallo", correcting to "hello" may be fixing an error or introducing an error. In general, the more aggressive your error correction, the more errors you inadvertently introduce. You can try and make a judgement based on context ("hallo, how are you?"), which certainly helps, but it's only a mitigation. Light error correction is common and effective, but you can't push it to a full solution. The only way to fully solve this problem is to look at the entire document at once so you have maximum context available, and this is what non-traditional OCR attempts to do.