I'm not following this logic.

It's the formula to calculate a character's Agility stat.

`lda $0431,x ;XP low byte`

lsr

lsr

sta $50

lda $0438,x ;defense

lsr

lsr

clc

adc $50

sta $50

lda #$80

sec

sbc $50

bsc $02

lda #$00

sta $50

lda $0430,x ;level

asl

clc

adc $50

sta $50

lda $042A,x ;character ID

tay

lda $8EAF,y

clc

adc $50

sta $0439,x

rts

So, going backwards, $8EAF is the character's base Agility (and it increases at 2xLevel), plus a bonus

where the bonus is

128 - Defense/4 - XP/4.

So, every 4 XP you gain, you LOSE 1 Agility point, until you reach 256 XP, then it rolls back.

(and the penalty is cleared at level-up because XP is reset)

I have to wonder if this is a typo or if maybe if one game designer gave the programmer some handwritten notes and they misread 437 (attack) as 431 (XP). Because lower Agility based on attack and defense makes a lot more sense as a crude way of adding a weight system to the game (for a 1987 JPRG).

I have another untranslated line

どうか ごぶじで......

The princess of Fahrei says this if you speak to her after you give her the magic medicine to cure her illness.

はしを わたり

コルテスに ゆかれるが よい。

Pretty sure this is "You may cross the bridge to Koltes." (the king is giving you permission to cross the bridge to the next part of the game)

EDIT: For the first line, I've decided to use a Google suggestion and put it as "Please stay safe...". At least I'm assuming she wishes you a safe journey.

Another one:

さなだ じゅうゆうしきんじつ じょうえい。

Sanada Juuyuushi

Playing soon for a limited engagement!

Not sure if I should go with a direct translation (Ten Braided Braves?), romanization (Sanada Juuyuushi) or replace the reference, Spy vs. Spy maybe? (Superman's an obvious joke, so much it's already been taken by another line)

おれに ようかい。

Random NPC.

めかしこんで デートかい。

Does this (perhaps gay? :) ) NPC want to date me?