Custom Multi-select fields can be added by clicking on the + button in the Global or Local Fields manager, and by selecting the field "Multi-select".
It functions similarly to the "Select" field, however with this option your participants are able to select several values.
Planners can edit the internal name of the field (e.g. Languages spoken), the label which is what your Participants will see (e.g. Languages you can speak), and the category the field belongs to (e.g. Personal information).
You can then add Values to be selected from - Multiple values can be selected by the participants.
You may also translate each value: Select the value, and type its translation in the relevant language field.