This tab appears only if to set value in tab Settings -> Languages. On the screen above value is: en_US,ru_RU,es_ES.
In the tab "Vocabulary" it is possible to translate words which you applied in the tables columns and also taxonomies terms. In behind the scene it works through function TABLEON_Vocabulary::get($word) which can be used in extensions.
To add a word into Vocabulary use button "Create" and write there exact word or sentence you want to translate. To find any word quickly use text input on the table top.