How to change the "W" (English) to "O"(French) on the North Arrow direction using ArcMAP 10.0? I tried to convert the North Arrow sign to graphic but it didn't work.
The fonts are images that need to be edited outside of ArcGIS, or you can use fonts without letters and add the letters yoursefl as text elements.
French fonts for the North arrows are availale on the Georezo forum after registration. http://georezo.net/forum/viewtopic.php?id=41480 . Maybe their author is on this site too.
ESRI North Arrows are stored in a Font file on Windows in C:\Windows\Fonts, 'esri_40.ttf' or 'ESRI North Regular' as labelled. You'd have to modify or adapt the file there to change the font file for your purposes, though I am not sure as to the licensing behind the north arrows and whether or not you can use them as modified.
You may consider contacting ESRI support to see if they have something already made up somewhere, or using a different set of North Arrows.
If your map is oriented with north at the top you could create an emf graphic using ArcMap as your drawing/design tool. You can add the image of the north arrow you want as a raster and using it as a guide digitize (draw) the north arrow using polygon, polyline and point shapefiles. Add annotations for font and move as needed. EXPORT final product to EMF file and add graphic to map.
I use this procedure to create complex north arrow that are set to a certain magnetic declination. So although this is more cumbersome then simply adding a north arrow (which is a font as ESRI is yet to figure out how to allow users to use vector graphics) is is possible to design your arrows at a specific azimuth by setting the the rotation of your data frame.
Here is an example of my typical "custom" north arrow:
A quick fix would be just to add a white graphic box in your layout over the "W" and put a "O" (same size and font style) text element over that spot. Ideally, you could export out this image and make your own picture to insert for other maps, however I'm not sure how that would work on the licensing side of things.