~ I truly believe that Brynjolf is afraid of commitment and his own feelings. Really expressing himself scares him. He rarely does it. So his love language will definitely not consist out of talking. Don't expect him to throw I love you's your way. Sorry lass/lad but he has more important things to do.
~ Brynjolf feels bad for not treating you like you deserve it simply because he has trauma and is scared so he'll come up with a dozen other ideas.
~ It will start out with a lot of praise. He tries to be charming and witty like always but it usually sounds rather... uncomfortable. Oh, you succeeded in your latest contract? Well done. *pats your shoulder awkwardly*
After he pats your shoulder he has a mental breakdown while overthinking it and makes plans to move to Markath and never speak to you again. He also gets drunk and proceeds to annoy everyone in the guild.
~ His second idea of charming you is to keep you away from drama in the guild. Vex has been messing with you? Well big hero Brynjolf is here to save the day. He will probably end up either annoyed or hurt because Vex is messing with him as well.
~ After a while, when he gets more or less comfortable with his feelings around you he'll make a lot of (flirty) jokes. While he still is nervous around you, he is now accustomed to it and he knows how to act like he isn't.
~ As soon as he is ready to get more serious, his jokes will get less flirty and more serious, complimenting you in some way. He will recall a lot of nostalgic memories, trying to see if you smile at the thought of them as well.
~ Brynjolf will start doing tasks for you. Cooking meals, going to get (steal) groceries, you name it. His sudden will to help surprises everyone who doesn't know him enough to realize what it means. Vex and Delvin definitely know what it means. The rest of the guild doesn't care or is confused.
~ You want to go for a walk? He'll join you. Like a gentleman he offers you his arm and if he's in a good mood and you bribe him a little he might even read something for you.
No matter what you're doing. Brynjolf wants to follow you and spend some time with his favorite person. Sometimes he even plans little trips on his own. They mostly happen at night and involve stealing something but he puts a lot of thought into them. He can get romantic as well. I can totally imagine him inviting (forcing) you to swim with him in the lake at night. How romantic. *swoons*
~ Bryn starts giving you little gifts. Gems, food, your favorite drink, a sweetroll, everything your heart desires. You know that he steals them but he risks his freedom for you so treasure it.
~ After some time he will start to open up. He'll talk about his past and his interests and if he sees that you're interested in what he's telling you, he'll fall in love even more.
~ I 100% believe that Brynjolf loves children and visits the Riften Orphanage multiple times during the week. He'll take you with them and introduce you to his kids. He won't tell you that he taught them how to pick locks.
~ As soon as he knows that you feel the same as him his teasing will start. You are visiting him because you missed him? How pathetic. That wouldn't happen to him. Nope.
~ There are moments where he gets sentimental and emotional, mostly when he's drunk, and suddenly he can't stop talking about how amazing you are. He even smiles instead of smirking like he usually does. Imagine you're sitting on the docks in the summer, feet dangling above the water. Brynjolf smiles softly at you as the sunlight reflects in his green eyes and makes them sparkle mischievously.
~ He will brag about you. Oh you have children? Well his significant other is the dragonborn bitch. He won't be able to stop smugly grinning for the remains of the day.
~ Physical contact is important as well. Sex is an important way to tell you how he feels.
~ Brynjolf only wants the best for you. He respects your wishes because he'd do anything to make you happy even if it hurts him.