Ah, ussage is your promblem! this is how you are supposed to use it:

at setup time call sp_addmessage for all required languages

ex:

use master

exec sp_addmessage @msgnum = 50001, @severity = 10,  @msgtext = N'US English message',

   @lang = 'us_english'

EXEC sp_addmessage @msgnum = 50001, @severity = 10,    @msgtext = N'Mensaje en Español',

   @lang = 'Español'

immeadiately after the session is stablished

execute this:

set Language 'Español'

then all your raiserror statements will be returned in spanish

if instead you do

set Language 'us_english'

then all your raiserror statements will be returned in English

make sure your numbers match and are greater or equal to 50000

hth