Providing translations for the labels and descriptions of your actions is much the same as for field labels and descriptions.  You just have to follow the naming conventions in your language .ini files:

actions.%action_name%.label

The label of the action %action_name%

actions.%action_name%.description

The description for the action %action_name%

Example

[print_reports]
url="$this->url('-action=print_reports')
label = "Print reports"
description = "Print the stock reports for the day"

Now, for your french users, you may want the label of this action to be "Rapports d'impression", and the description to be "Imprimer les rapports courants pour le jour".  All you would have to do is add the following to your fr.ini file:

actions.print_reports.label="Rapports d'impression"
actions.print_reports.description="Imprimer les rapports courants pour le jour"