Displying % alcohol in product display

Hi, I am selling alcoholic beverages and would need to display the strength of the liquor in percent alcohol. To do so, I overwrite product length in the language files with Alcohol (which works fine when displaying) and tried to specify the length symbol as "%" instead of "m". Unfortunately neither "%" not the word "percent" is accepted. How can I solve this to comply with the law?

solved... I used a language overwrite M="% vol."