I’ll gladly work with librariesel to help finish translating the French if needed.. and it would be good indeed that we don’t all have to do our own translations on our own.
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Apart from this I’m a bit confused with the issue/fix treated by Karel..
Three points regarding this:

1a.
In my install of Neighborhood the language folder is in the singular not plural.. I see some themes calling it lang some others languages.. Isn’t there a “best naming practice ” standard for those language folders that the WP makers indicate ?

1b.
Could the child-theme languages folder with an s “confuse” WP at all considering the above ?

2.
Should one only upload the required .mo .po in the child theme languages folder (in my case French as librairiesel) and leave the other languages in the original one.. or is the way Karel exposes possibly better, namely put all .mo .po files in the child theme ?!

3.
After updating the theme possibly, not sure when that happened.. I now have the following or US english:
en_US.mo
en_US.po
en_US_old.mo
en_US_old.po

Can I safely delete these two “old” ones ?!

T H A N K S !