No I downloaded the plugins from the sourceforge depository and compiled it. I wonder if it detects the system localisation and compile a version in the country language... I hope not.
My Ubuntu Server is in English, as is the result from the ping command...
STOP, I found out. Sorry, I was searching while responding.
There is some files in the directory /usr/local/nagios/share/locale that gives translation for the plugins messages.
Those files are created (or it seems so) using the files in the "po" directory of the plugin package.
Now that I know where the translations are made, I'll try to find a solution... maybe delete the translation file.
I'll keep you posted.