1) How can I change the default language in Zotero?
By default, Zotero will use the same language as your operation system (i.e. Windows, Mac, etc). Please refer to here to change the default setting in your Zotero.
2) How can I know which fields in Zotero I should input for different types of references (e.g., journal article, webpages)?
Although Zotero is a convenient tool to manage and insert citations, you still need to get familiar with the citation style that you are using. You can refer to the Referencing Guide from ELC (English Language Center) below for more information: