This lovely poem addresses an interesting question that is on one level a complex linguistic problem and on another level a very personal problem. The theme that emerges from the examination of both of these levels is that while limited language is a barrier to self expression, the love of the heart climbs the barrier and gives a metaphoric--and idiomatic--hand up, an idea that focuses attention on the first stanza.
In the first stanza, the poetic speaker (which may or may not be the voice of the poet) addresses the difficulty a nonnative speaker has in navigating the waters of cultural idioms in a learned language. Idioms are a particular sort of linguistic problem in learned languages because idioms are based on figurative concepts in the culture that are widely shared and understood but are not shared by other cultures. For example, if I say, "It was high and over the fence," an American gets the sense that I'm making a baseball allusion that means that something was done with great skill and even greater success (high and over the fence refers to a powerful home-run hit that goes out of the field over the back fence) but people in other cultures that don't follow baseball (are there any left?) won't know the reference and therefore won't know what I'm trying to say.
Conversely, if a speaker of English as a learned language tries to use the above idiomatic expression, the expression or the intent may get jumbled and the culture-dependent, culturally understood meaning get lost so that the listener has to try to sort out a new meaning. For example, if I change the above expression to, "I went high and over the fence," we're not sure what it might mean: I did something successfully or I jumped and climbed over a fence? This is what the poetic voice calls "clumsy on the tongue, these acquired idioms, / after the innuendos of our own": idioms are clumsy because the cultural background giving them meaning is missing; they have "innuendos" because of imperfect understanding generating original, non-culturally determined, usage, as in the example above.
The second point made is that the inadequacy of language mastery prohibits or at least limits expressing one's true or full thoughts. Linguists debate whether or not it is a truth that all languages are equally expressive of all types of thoughts. Reid is indicating in this poem that from an experiential perspective the truth is that not all thoughts can be expressed at all levels of language mastery, which means vocabulary acquisition, grammar comprehension, syntax mastery. This gap between thought and expression, this gap of things "lost in translation," is described as "always wallowing / between what we long to say and what we can."
The second stanza comes to the rescue by insisting that love allows the listener to reach out in the simile of a "helping hand," or "limp in sympathy" to guide the nonnative speaker to a safe harbor, as it were, of understanding, a safe harbor away from the "tangle of language" for the heart "groping toward" them so that anything "being lost in translation" is reclaimed by the "translation of syntax into love." So the theme, delivered from the very empathetic listener's perspective, is that love transcends the frailties and inadequacies of as yet imperfectly learned language.