Learn Italian: Non la dimenticheremo mai, la sua lealtà. - We will never forget it, his loyalty.

Nouns ending in an accented vowel are invariable, meaning their spelling doesn't change in the plural. To say 'the loyalties', you only change the article to make it plural: le lealtà.

Why does the sentence use the pronoun la before the verb when la sua lealtà is already mentioned at the end?

This is a common way to add emphasis in spoken Italian. By using the pronoun la (it) before the verb and tagging the actual object, la sua lealtà (his loyalty), at the end after a comma, the speaker makes the statement much more dramatic. Literally, it translates to 'We will never forget it, his loyalty.'

Why is there an 'h' in dimenticheremo? The infinitive verb is dimenticare.

Verbs ending in -care and -gare add an h in all future tense forms. This is done to keep the hard 'c' or 'g' sound. Without the h, the 'c' followed by an 'e' would mistakenly be pronounced with a soft 'ch' sound.

Why use Non ... mai instead of just saying mai?

In Italian, when mai (never) comes after the verb, you must put non before the verb to complete the negative meaning. While you can occasionally put mai before the verb without non, the non [verb] mai sandwich used here is the standard, most natural way to say 'never'.