Update first Merge Tutorial Video; Add French subtitles

[imnasnainaec]

Part of #2776

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.67%. Comparing base (293e202) to head (a7e2dd1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3514      +/-   ##
==========================================
+ Coverage   74.64%   74.67%   +0.03%     
==========================================
  Files         285      285              
  Lines       10995    10995              
  Branches     1337     1337              
==========================================
+ Hits         8207     8211       +4     
+ Misses       2404     2401       -3     
+ Partials      384      383       -1     

Flag	Coverage Δ
backend	83.62% <ø> (+0.07%)	⬆️
frontend	66.71% <ø> (ø)

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[jasonleenaylor]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)