Learn Romanian: Suntem loiali și ne sprijinim unul pe altul. - We are loyal and we support each other.

You are right that ne sprijinim usually translates to 'we support each other.' However, ne is also a reflexive pronoun, so it could theoretically mean 'we support ourselves' individually. Adding unul pe altul emphasizes the reciprocal action and removes any ambiguity.

How do we know to use unul pe altul instead of the feminine una pe alta?

We look at the adjective loiali. It is in the masculine plural form, which in Romanian is used for all-male or mixed-gender groups. Because the group includes at least one male (or we are speaking generally), the reciprocal phrase must be the masculine unul pe altul.

Why is the preposition pe used between unul and altul?

The verb a sprijini (to support) takes a direct object. In Romanian, when a direct object is a specific person or pronoun, it must be preceded by the preposition pe. So the phrase literally structures as 'one (supports) pe the other'.

How would this sentence change if an all-female group were speaking?

If the group were entirely female, the adjective would change to the feminine plural loiale, and the reciprocal phrase would change to match: Suntem loiale și ne sprijinim una pe alta.