Maybe ask these guys https://geneve.unia.ch/

As this is your first job and you have been working less than one year, the "code des obligtions" (Swiss code of obligation) states that you have to give one month notice to your employer-which means that you could have left on January 18, 2019.

Both the employer and the employee can however, immediately terminate their working arrangements for "good reasons" (these are however, exceptional)

It appears that your employer feels that you have unjustly abandoned your position (abandon injustifié) and as such the law states that he/she is entitled to deduct 1/4 of your monthly salary as well as additionlal monies should your employer be able to prove that she/he has incurred negative consequences as a result of you leaving your job.

https://www.vd.ch/fileadmin/user_upload/themes/economie_emploi/emploi/fichiers_pdf/FAQ-droit_du_travail-SDE.pdf

You can file at no cost of your own, a claim with the court, "tribunal des prudhommes" if the total prejudice you incurred is less than CHF 30,000.

http://www.lausanne.ch/en/lausanne-officielle/administration/sports-et-cohesion-sociale/service-du-travail/protection-travailleurs/travailleur/litiges-contrat-travail-guide/formulaire-requete-conciliation.html

Hoping this information is of help, Karin

It's usually one month notice for the end of a month. It is not one month exactly. Therefore, 18th December 2018 = 31st January 2019. The 13th salary can come with conditions and could be withheld. Your best bet is to bring your contract to an organisation such as Unia for clarification on that and on abandoning your post.  

Hi, for 50.- you can get a consultation from a lawer in the old town (permanence de l'ordre des avocats) and they could help you draft a letter or tell you your rights as per Swiss Law.

Good luck!