Break "ar lath ma, vehnan" down a bit, and it becomes a meta goldmine.
Ar, often used as the personal pronoun “I”, except in “ar lasa mala revas,” where it seems to function as the personal pronoun “you.”
Ma has a similar ambiguity; it can mean either “you” or “me.”
She is his heart, and he loves her—the literal meaning of the phrase—but he’s also playing with the variations of meaning their language allows. Implicit in the phrase, with its ambiguous pronouns, is the declaration that love blurs the boundaries between their two selves.
I love you, heart. You love me, heart. Both of these readings exist within that sentence, symbolically turning the two lovers into one being, for whom the distinctions imposed by personal pronouns aren’t clear. Then he tops this off with “vhenan”—heart. Not “my heart.” Just “heart.” He claims no personal ownership of it. In the context of the multiple meanings this sentence has, his phrasing suggests that these two selves, merged as one through the linguistic wizardry of the elven language, have exactly one heart between them.*
Romantic, yes. Also curious. How straightforward can anything Fen’Harel says be?
My guess: not very (because it has to be "more complicated than that," right?).
Exhibit A: The smoke around the Inquisitor after she topples out of the fade in the intro, smoke that seems to be the visual clue for spirit swap, at the very least (see, Well of Sorrows, and the end of Flemythal; smoke visually signifies the spirit transference taking place).
Exhibit B: the mark, fused with the Inquisitor, which sends Corypheus into villainous ranting about the Inquisitor’s designs on godhood. Whatever else the orb may have been capable of, the only thing left of it by endgame is the anchor, now inextricable from the Inquisitor’s being.
Exibit C: Solas’s concern that the Inquisitor’s spirit might have been affected by this merger. Which he now knows has taken place, if not because of his own connection to the orb and study of the mark, then because of Cory’s attempt to wrench back the anchor. He asks this question on the balcony, before the kiss that leads to his declaration, and it appears to be a particular concern.
He professes his love, yes, but I think he’s simultaneously coding a message about their actual spiritual state. It wouldn't be a Solas statement otherwise. Think about it: pretty much everything he says in game functions on a couple of different levels. He is the undisputed master of the hidden meaning and half-truth. In this instance, however, he's using his native language to do it. The phrase he uses reflects the multilayered ambiguity of the language in which it is spoken.**
If the orb was sacred to him him, and it was (he himself says these objects were dedicated to “some of our pantheon” during the veilfire conversation after Haven’s destruction), then it’s not farfetched at all to suppose a mystical connection between Fen’harel’s essence and the orb. And to get at the essence of something is to get at its heart.
In a manner of speaking—again, that linguistic sleight of hand—the anchor is his heart. His heart beats in her hand, permeates her being. They are two, but they are one.
It’s not just a simple “I love you” he’s delivering. He’s also telling her that they are actually linked. Spiritually, magically, however the orb works, Fen’harel and the Inquisitor are now connected. The love he feels, bittersweet as it may be, is just the most observable aspect of that connection.