I am not sure how to report bugs, or even if anybody is interested in bug reports. However, I have been playing with lucene lately, and found out an implementation bug in the Frenchstemmer (/src/contrib/Analyzers/Fr/FrenchStemmer.cs). Whenever I tried to add a new document to an index, I got an index out of range error. So I looked at the code and fixed that issue: see my diff file attached.
Please note that I also changed a few funky characters to unicode notation. The code worked well with the funky characters, but I think it just looks better with the \uxxx bits...
Anyways, the important bits is the replacement of a couple of sb.Insert by sb.Append.
I hope this helps.