Si un article n’a pas de code-barre, je peux cliquer sur le bouton qui dit, « Rechercher le nom de l’article » .

Lorsque je vais au supermarché, j’aime utiliser les caissies automatiques.

- I guess "cassis" was a typo. The word exists, it is a kind of black berry that faintly smells like cat pee, to my nose at least. I think the English name for it is black currant.
- "Lorsque" is perfectly fine, just mind the fact that "Quand" might be preferred in oral speech as it is more casual.

Au Canada nous pouvons choisir de voir les instructions en anglais ou français.

"choisir des instructions" : pick instructions
"choisir les instructions" : kinky... actually what you chose is not the instructions themselves, but the way they appear

alternatives :
"nous pouvons configurer la langue d'affichage des instructions entre français et anglais"
"nous pouvons choisir d'afficher les instructions en anglais ou en français"

Je choisis le français car je suis en train d’apprendre cette langue.

The sentence is good, but it might lack a precision as to the "when".
"En ce moment je choisis ..." nowadays
"Je choisis parfois ...", "Il m'arrive de choisir ..." sometimes
"Je choisis toujours ..." always

Sur le premier écran il dest écrit, « Bienvenue - Commencez à scanner vos articles » .

"il dit" ~ he says. That is not really wrong, many people would have a tendency to personify a machine or a computer for instance. My mother would totally call me to ask what to do when the computer is block, and saying things like "Et maintenant quand je clique sur entrée il me dit ...". That being said in the present text we haven't formally been introduced to the machine you want to personify, unless we have to go back to the title, and then it would be une caisse automatique => "elle dit".

"il est écrit" ~ it is written

Si un article n’a pas de code-barre, je peux cliquer sur le bouton qui dit, « Rechercher le nom de l’article » .

In french we click *on* the button... Thanks for reminding me, I am sure I would do the opposite mistake in English :)

Je l’ai essayé une fois, mais je ne connaissais pas le mot français pour « ail » .

To make an attempt is "essayer" without an object : "je n'ai pas essayé"

"je l'ai essayé" indeed means "I tried it", where "it" is something concrete, like an object. Exemples : "Tu as vu le nouveau tracteur du voisin ? Oui, je l'ai même essayé.", "Qu'est-ce que tu penses des nouvelles Adidas ? Je ne sais pas, je ne les ai pas encore essayées."
You could argue that the button is an object, so you could try it... But this is not our logic. In french we wouldn't really try the button, we would try to use it or to push on it, therefore an action...
Actually, I can see a case where you could say that with a button. Let's say there are three buttons doing specific things... a blue on the right giving treats, a green on the left controlling music, and then there is the big red one in the middle that you never dared to push yet. In that context you could say "Je ne l'ai pas encore essayé", as here, clearly, « l' » refers to that specific button as opposed to the others.
In that context : "Je n'ai pas encore essayé" would still hold, and "Je ne l'ai pas encore essayé" would add some info such as ("I already tied the others" or "I am impatient to try".).

Donc j’ai dû cliquer sur le bouton que a dit, « Change to English «» .

"que a dit" doesn't exist,
"qui a dit" ~ "which then said" / "who then said"
"qui disait" ~ "which said" / "who said"
"qui dit" ~ which says" / "who says"

In french we could say something like "le bouton qui disait « feu !» .", although formally we should rather say "le bouton sur lequel était écrit « feu !» .". That being said the shortest and the best would be "Le bouton « feu ! ».".