l10n: Update the method for inserting FTL resources into the main window

We need to check whether this approach is better:  https://github.com/jlegewie/beaver-zotero/commit/635e94ebf13d5ad138c7a02ee498a44c7c8dec01

ref:

- https://github.com/jlegewie/beaver-zotero/issues/162
- or https://github.com/zotero/zotero/issues/5755