How to convert a string to their IPA equivalent

You're going to have at least two problems with this which will limit the size of your word lists and/or the number of languages:

1. IPA is not a perfectly universal standard directly representing each sound's phonetic qualities without any regard to phonology. Each language has its own conventions and each transcription has a level of detail encoding that is down to the individual decisions of the encoder. (E.g. the different pronunciation dictionaries of English do not actually perfectly correspond to each other.) So you're going to have to create your own basis of comparison and translate the IPA encodings used for a particular language into that. That will be massively time consuming the more languages you look at because you're going to have to actually research what the IPA means exactly in that language - just looking at the IPA will not be enough. So practically, you can really only do this if you already know what you're looking for (at least the area). Also, do not forget that IPA will not capture the different types of variation present in any language.

2. Because of this, there are no tools that can do this universally or reliably for most languages other than English. Many languages have sufficiently regular orthography (Czech, Greek, Spanish, etc.) that they don't actually need to encode their word lists into an intermediary standard such as IPA. In fact, the IPA you see in English dictionaries is just a substitute for such an orthography rather than a phonetically accurate representation. This means that it will be relatively easy to construct a simple IPA converter for most of these languages - but all the caveats from point 1 still apply. This will have been done for languages that have text-to-speech technology but there's no open source list of these that I know of. But you will still have to convert these into a system that makes them comparable for your purposes.

For English, this has mostly been done and there are a lot of online tools. I've used Photransedit which gives decent (but not 100% reliable) results. But you can also get word lists annotated with IPA such as the [MRC database][2] (which uses a machine readable format with a 1-1 correspondence to IPA (done before SAMPA).