Can't find "Spellchecking and Writing Aids"setting in Language support anymore in Ubuntu 12.4, how could i change this setting now ?
For system wide language go to "system settings > Language Support". For individual programs like libreoffice, open libreoffice and go to "tools > options" which brings up a window and under libreoffice "Language settings > Writing Aids" it has a checkbox for checking while you type.
Press Super then type Language, you should see it:
Hit Enter to launch it. Then you will see the "Language Support" app:
To disable the check spell at specific apps you will have to change the app configuration.
For example, at Firefox go to "Edit" > "Preferences" > "Advanced" > "General" > "Browsing" and uncheck "Check my spelling as I type":
And at Libreoffice: go to "Tools" > "Language" > "For all text" > "None".