Where does “ბრიტანეთში” come from?

ბრიტანეთში (Georgian) comes from Georgian -ში, from Laz -შ, from Proto-Georgian-Zan ḳwal-.

Ancestry of “ბრიტანეთში”, step by step

StepLanguageWordMeaning
1Georgian-შიin, into
2Laz-შforms nouns from adjectives meaning “the one having such a quality among the same category of things”
3Proto-Georgian-Zanḳwal-—
Every word from Proto-Georgian-Zan ḳwal- →Every word from Laz -შ →Every word from Georgian -ში →