もっと強くマッサージしてって英語でなんて言うの？

▪ Can you massage a little harder please.
,,,,,,,,,,,,,,
Harder is to do it stronger.
This sentence is asking/telling the masseuse to massage harder/stronger.

Example
Masseuse: Is this okay for you?
You: Can you massage a little harder please.

Since to 'press' is to push something firmly, you may ask the masseuse to press harder on your shoulders; back, e.t.c. It's also probably a good idea to use words like 'can' and 'please' in order to sound polite.

This means that you would like them to massage you a bit harder. You can also tell them which part of your body you would like them to concentrate more on so that you can have the best experience.

examples
"Can i have a firmer massage".
or
"i would like a stronger firmer massage please".
or
"can i have a harder firmer massage please".

When you're asking the person massaging you to rub a little harder, then you can say:
-Please press a bit harder there (on my lower back)
-Please rub a little stronger (on my neck)