Could we have this text (string 139) in bold and/or blink and also showing the label like
Warning, this path is a subdirectory of the existing folder "{{otherFolderLabel}}" ({{otherFolder}}). ?
We also have an issue here: https://github.com/syncthing/syncthing/issues/3433
You can make a pull request yourself.
I can’t : when I clic new pull request I get a compare button I know what to do with like a fish with a hammer. I’m not a coder.
You can edit files from github, which will cause you to create a fork, which then will prompt you to create a pull request.
Thank you for help man. It seems I have to learn git, what I really no have time enough for, although it seems to be a great tool for real coders (I’m just casual, and I won’t have presumptuousness to walk in real coding). Do you know if I just can add html tags like [b]Attention…[/b] in Transifex string ? I know it wouldn’t be the best way, but it’s my level 
Also, for blinking, some searches over the web convinced me it is a bad idea (support removed for good reasons out of the few web browsers that previously supported it).
Here is the string I did for this warning, as a result of my recent experience with a ST newb friend : Attention, ce chemin est un sous-répertoire du partage existant “{{otherFolder}}”. Ceci peut causer des problèmes tels que duplications de fichiers ou suppressions intempestives sur les autres machines. It might be worth updating the English string about the reasons of the warning (…May cause dups and unwanted deletions on remote machines)