Special characters will mess up the form so they need to be URL encoded. Keep in mind that you are building a URL. It's not the length that is the problem as the URL can be extremely long. Internet Explorer most restricted in length but its limit is still over 2,000 characters long.
Try this
var text URLONE = URLRoot() & "db/" & Dbid() & "?act=API_EditRecord&rid=" & ToText([Record ID#]) & "&_fid_47=" & ToText(Today());
var text URLTWO=
"MailTo:"& URLEncode([Store No - Contact Main Email])
&"?subject=" & URLEncode("Store " & [Store No] & " POS Checkout Site Readiness Survey and Install Guides / Guides Sondage de pr�paration du site et installation pour les caisses enregistreuses")
& "&cc="& URLEncode([Store No - Contact Secondary Email])
& "&
bcc=checkout@cantire.com;
RSSECLEVEL@cantire.com"
You will see that I URLEncoded the Subject line. I removed the from.
I tested it and that worked - almost. The issue will be the � in pr�paration. Quick Base is not great at handling alternative character sets. So you may have to drop the � and go with e to get that to work.
Sometimes in French it is easier to get away with dropping the accents if the words are in caps. ie you may need to ask your French support staff which is the least worst, just going with e or making that whole subject line in UPPER CASE.
If you need to remind the users to change the "from" manually in Outlook, then you can add some instructions in the body like
& "&body="& URLEncode("attention: remember to remove this note and change the from to xxxxxx and remember to attach the readiness Survey")