What language technique is in the quote "Link, that boy might go to the chair, but he's not going till the truth's told...And you know what the truth is"?

The most obvious language technique in this quote is alliteration, which is when words in close proximity begin with the same letter and/or sound. In this case, it's the consonant t and the phrase "'till the truth's told." Till, truth, and told each begin with a hard "t" sound. You wouldn't include "the" in the alliteration pattern because "the" is a th-sound as opposed to a hard "t".

More lightly we see the use of a euphemism, which is an indirect word or phrase people use in place of something that's harsher or blunt. "Go to the chair" does refer to a physical chair, but more often people use the phrase as a light way of saying that someone's going to die via electrocution. It's unpleasant in conversation to refer to the death of another person, so Atticus uses this phrase to make the conversation seem a bit less heavy to both Link and those who may overhear.

In this passage from Harper Lee’s novel To Kill a Mockingbird, Atticus Finch is justifying his defense of Tom Robinson to Link Deas, who has questioned Atticus’s motives on moral grounds. The language technique used in this passage is alliteration.
Alliteration is defined as the repetition of the initial letter of a number of words in a phrase or line. An example of this is “pied piper.” The purpose of alliteration is to create emphasis.
The example of alliteration in this passage comes in the following quote, “till the truth’s told.” Each of these words begins with the consonant “t.” The use of alliteration here illustrates the importance of the truth in the case of Tom Robinson. For Robinson, the truth really is a matter of life or death. Therefore, using the alliterative construction within this passage underscores this reality.