To update a translation file call
> lupdate @l10n_file_list.txt -ts ts/app_<LANG>.ts

Then use the linguist tool to translate the strings in the .ts file.
linguist ts/app_<LANG>.ts

The l10n_file_list.txt was generated using the following command.
> find .. -iname *.[hc]pp | sort > l10n_file_list.txt
