You will obviously experience multiple deficiencies that may not be obvious until you go on a 14 day cruise where your body finally gets to enjoy a variety of foods and realise how ridiculous your diet and this question was.
“Mostly” is a fairly weak term, here; it doesn’t take much space in a diet to patch up the holes, and survive until something else kills you a fair amount of the time. Lentils are an outstanding staple food, and you could do pretty well if you knew what to do. But people vary; you might have some digestive complaints. Also we absorb nutrients differently, and lentils can block some.
A very strong move would be to eat sardines (not boneless) regularly; perhaps spaced apart from the legumes. I don’t eat a lot of dairy, but I would add kefir maybe, and more cheeses. Hopefully it’s decent soup; I’m hoping for some good fats and herbs, and meat would be nice.
Nothing good can come of this.
Man wasn’t meant to live to any dietary extreme.
If you can eat only lentils for more than a year or so without committing suicide, you got skipped in one of the chromosome lines.