Sometimes after adding a new database field to an Extbase record in a multilingual TYPO3 website and setting a value for this field in the default language record, we want this same value to be also available for this field in our Fluid templates for translated records without having to edit and saving all the translated records in the backend.
To do this, set l10n_mode = exclude at the TCA definition for the field
This just makes the value of the field in the default language to be copied to the field of the localized record.
A full explanation for this TCA property can be found at time of writing at https://docs.typo3.org/m/typo3/reference-tca/main/en-us/Columns/Index.html#confval-columns-l10n-mode