But if you want to incorporare reaction rolls and keep it simple, you have options:
Take One: Assign predispositions to enemies in advance. Most are irredeemably hostile, doubtless explaining why so many have abandoned (or otherwise ignored) the idea straight away. Those foes amenable to bargains or bribes can be identified in advance, with terms and conditions spelled out. The Goblin Chief is wicked, but appreciates bribes of 100 GP or more per the GM. This is implied in certain rulesets and bears elaboration.
Take Two: Once again; the above, in some form, is close to standard practice, with countless iterations befitting the milieu. Another approach, for the adventurous, is to rework a game's entire system, merging combat initiative and enemy reactions in a single roll. One player rolls for the entire party (a rotating duty) with enemy reaction and initative as indicated:
2-4 Hostile; enemy takes initiative and attacks immediately
5-7 Negotiable; monsters hold initiative, but may pause for sufficient overatures
8-10 Hostile; party holds initiative and may attack or attempt to bargain
11-12 Negotiable; characters win initiative, but the enemy displays hesitation
Of course, any openess to negotiation is lost with hostile action from the party, noting that superior players will already know to inquire about an enemy's non-verbal cues, saving themselves needless bloodshed. The GM will still need to establish temperment in advance, adjusting accordingly. This approach has the added benefit of being easy to remember in battle, and easy to tailor to conditions. Reactions are only positive when conditions are met, meaning that Take One (above) still applies. A great choice for Mydwandr.
Some enemies are always hostile, others always amenable to negotiation. In these instances, dice indicate potential attack order only. The rest comes down to the fine art of game mastery. That's because while the dice can generate outcomes, only a seasoned referee can translate entries on a table into a natural and seamless narrative where player decisions matter. Reaction tables are old-school because old-school rests on a sandbox stocked with random outcomes seasoned by strategy. But it needn't be difficult...