Monitor trademark registers in the countries where you hold registrations and where you are commercially active. These are the jurisdictions where you have enforceable rights and where an infringing application could directly threaten your position.
Beyond registered markets, monitoring in countries where you plan to expand is advisable: a third party registering a similar mark there before you file would create a conflict that is harder and more expensive to resolve than it would have been to prevent.
For online channels (social media, marketplaces, domains), monitoring is not jurisdiction-specific in the same way; infringement in these channels can affect you globally regardless of where your registrations are held.