feat(ui): Add language support for kopiaUI (#3768)

* Add multi-language support

* Adding a guidline to outline the localisation process.

* Moved doc to separate pr

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Co-authored-by: lupusA <lupuapps@gmail.com>

internal/serverapi/serverapi.go

@@ -294,4 +294,5 @@ type UIPreferences struct {
 	Theme                  string `json:"theme"`                  // Specifies the theme used by the UI
 	FontSize               string `json:"fontSize"`               // Specifies the font size used by the UI
 	PageSize               int    `json:"pageSize"`               // A page size; the actual possible values will only be provided by the frontend
+	Language               string `json:"language"`               // Specifies the language used by the UI
 }