Dictation language changing randomly to Japanese which is not even configured in iPhone

Hello all, I have problem with my new Apple Watch 4.

Dictation language is changing randomly to Japanese language, which is not even avaliable in my iPhone keyboards or dictation settings. On the iPhone I use only Slovak and English language. For dictation I have only Slovak language selected on the iPhone (English is not checked), and I have never had any issues with dictation on the iPhone. However on Apple Watch dictation is switching to Japanese language, and I have to always force touch for change to Slovak language.

Do you know what may be causing this issue ? It is my first Apple Watch, and this bug is very anoying. I never used Japanese language on my iPhone, and I have no idea, why Aple Watch keeps switching to it for dictation.

I tried to reset Apple Watch to factory settings and restore it from backpu, but after one day, dictation in Japanese is back again.

Used hardware - Apple Watch 4 - watchOS 5.1.3 and iPhone 6 - iOS 12.1.3