If there is an agreement or company’s practice to provide the 13th and/or 14th salary, then it is legally bound to provide it.
In case where an employee was never paid the 13th and/or 14th salary and there is no relevant written agreement between him/her and the employer, then the employer is not legally bound to provide it.
If in the same company, part-time and full-time employees are employed, and the 13th and/or 14th salary is paid to the full-time employees, then it must be paid to the part-time employees proportionally.
Similarly, in cases of fixed-term and permanent employees, if in a company, the 13th and/or 14th salary is provided to a permanent employee then it must be provided to the fixed-term employee, as well.