Names, I think there's a list the config or something, and you can change them - but the voices will still sound however they normally sound. -- Not really sure, sorry about that.
Town names are one of two ways. In the config for the islands there is a long list of names. You can edit them there. But, I don't know how reliable that is as I've never tried it.
The sure-fire way to change the names of towns is to open the island (1st, de-PBO the island, get BIS's Binarize 1.0 and run it through there) in -- easiest is WrpTool -- an island editor.
Then, in WrpTool, there is an option to make new place names or edit/ delete them. Use that to change the names.
Signs - no. Not easily anyway. There are 2 textures for each sign name. One with the name in faded black. The other the same but with a big / in it. (For entering/leaving town signs). As far as I know, these are only stored in the "data" folder in the "dta" folder in your main OFP directory.
To edit these you'll need TexView (available from ofpec) and a decent picture program (Photoshop, PaintShop Pro, The GIMP etc.). Then, find a tut on how to edit textures. Or post back here with querries etc.
Hope this helps :thumbsup: