I was made some test with different hyphenation settings.
I didn't want override default hyphenation directories but used the method which is demonstrated in the thread:
https://www.mobileread.com/forums/sh...ion+dictionary.
So let's assume, a new dictionary, for example, a LibreOffice hyphenation dictionary is loaded to KOBO.
On the other hand, there is a good hyphenation plugin for Calibre, a long term used "Hyphenate this". I was started from this while it works with the uploaded language-specific hyphenation dictionary.
Then started playing with KOBO.
I was set the parameters for hyphenation as in the kepub formatting thread discussed. Sent my book from Calibre to KOBO using Hphenyte this! and watched the result.
Then removed the soft hyphens with the same tool. Set the language in metadata, put the book to KOBO. The results (in hyphenation) was not the same like before.
In the next step I set the language directly in epub in the <body lang="xx">.
With this setting, the hyphenation result was the same as with using the Hyphenate This.
That was enough test for me to conclude that it was hyphenated with my uploaded language hyphenation dictionary.
Just a note: if I was used the "Hyphenate This!" method I lost the search possibility on KOBO, while I couldn't type soft-hyphens in the places where the tool puts them.