Learn Swedish: Delegationen förhandlar om en internationell allians. - The delegation is negotiating an international alliance.

In Swedish, the verb förhandla (to negotiate) usually requires the preposition om (about/for) when you mention the topic being negotiated. It literally translates to 'negotiating about an international alliance.'

How do I know if delegation is an en-word or an ett-word?

You can see it is an en-word from its definite ending: delegationen (the delegation). As a helpful shortcut, almost all Swedish words ending in -ion (like station, information, and diskussion) are en-words.

Why doesn't the adjective internationell have a suffix here?

Because it is modifying en allians, which is a singular en-word in its indefinite form (an alliance). The adjective only takes a -t for ett-words (like ett internationellt företag) or an -a for plural and definite forms (like internationella allianser).