A lot of employment contracts in the Netherlands have a phrase similar to "werknemer dient ook alle voorkomende en/of in redelijkheid op te dragen werkzaamheden te verrichten" ("employee must also perform all occurring and / or reasonably assigned activities."), which is a blanket statement that an employer can require - within reason - an employee to perform other activities than enumerated in the contract.
There is also jurisprudence that an employer can require an employee to take a new function, if that function is on the same or similar level of their old function (the "Taxi Hofman Arrest" (Hoge Raad, 26 jun. 1998)). An employee can only reject such a function change if it is either unreasonable (e.g. requiring an employee to do something they are not physically capable of), or the level of work is entirely different (e.g. changing from a software developer to janitor work).
In addition, contracts usually have a unilateral "wijzigingsbeding" (modification-clause), which allows the employer to unilaterally change certain aspects of a contract, if and when the employer has a "zwaarwegend belang" (significant interest), and if the change is reasonable. They can't change your primary benefits (e.g. salary), and they have to state (and prove) their significant interest (e.g. reorganisation, bad results).
I am not a lawyer, nor an expert on Dutch employment law, but I think that an employer asking a software developer to switch to a different programming language is not an unreasonable demand (you're still a software developer, so it is same/similar work), and it is unlikely that a judge would agree with you that this is an unreasonable change if you'd bring this to court.
In short, you will have to accept this change in function, or otherwise find employment elsewhere.
If you really don't want to accept this new function, it might be worthwhile to not just give notice to your employer, but discuss the situation with them, and come to a transition agreement (transitieovereenkomst, vaststellingsovereenkomst) where technically the employer ends your contract with mutual agreement. The benefits are that you might be able to get additional months of pay, and - if you leave without a new job lined up - you will still be eligible for unemployment benefits (WW).