'Need' can also be translated by eisiau, although it is most commonly used to mean 'want'.
In the Tips and Notes says that "Dw i eisiau" actually means "I am + want", couldn't that be translated as "I am wanting". In this case "I am wanting a belt" ? Perhaps not idiomatic but grammatically correct.
The usual translation of Dw i eisiau... is 'I want...'. 'I am wanting...' is rather odd in English.
This pattern with eisiau is unusual in Welsh because eisiau is a noun (a want), not a verb-noun (wanting), and the expected 'n/yn after Dw i... is not there.
The pattern has probably come from a more formal pattern -Dw i ag eisiau xxx (literally, 'I am with a want of xxx') - from which the ag has been dropped in normal usage. Nowadays a different pattern has become more common in formal Welsh.