update variable from 7.1 to 7.2 : got error message "The form has become outdated"
So to answer the question :

for people who don't manage to update simultaneously variable and i18n to get variable 2.1 &amp; i18n 1.7 :

even if unusual, you have to update first from your current version (i.e i18n 1.4  and variable 1.1 ) to i18n 1.5 and variable 1.2, only after that you'll be able to update to variable 2.1 and i18n 1.7 versions.

Always following same steps :


clear cache
replace modules folders with the new version
run update.php