Devnagri script, which is used for a bunch of Indian languages including Hindi, is probably the simplest to transliterate. Each letter has a pretty well defined pronunciation with very rare exceptions. Vowels are completely regular and always have the same sound in all words with no exceptions (that I can think of). It should be pretty easy to write some simple logic in a few hundred lines of code to do this automatically... with the main difficulty being the target language rather than the source language.