Disclaimer - I am no contractor, nor a structural engineer.
You're right, ordinary Plexiglas or other acrylic in ordinary thicknesses will not support 40-70 kg over a 2 m span. But, AFAIK, neither will most other flooring. Here in the US, floor joists are typically 16" apart and spanned by 3/4" plywood or similar material.
You're trying to accomplish something rather special. You CAN secure glass in thickness of more than 2.5 cm, but it'll cost you, it'll be very heavy, and it'll require some special consideration in construction.
Just a thought, but if you're trying to achieve a glowing floor, as opposed to a transparent floor, maybe you should think about highly diffusive materials to spread the light "around" the subflooring supports. I can envision a torsion box design that would probably meet your needs in this fashion, but a transparent floor is another kettle of fish.
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