A silent mouth is sweet.
Note: Silence is golden. — English Proverb. Sprechen is silbern, Schweigen is gelden. (Speech is silver, silence is golden.) — German Proverb. If a word be worth a shekel, silence is worth two. — Hebrew Proverb. Is minic a bhris béal duine a shrón. Is minic a ghearr teanga duine a scornach. Cùm do theanga ‘nad chuimse. (Keep your tongue in hand.) — Scots Gaelic Proverb. The mouth is the tongue’s prison. — Arabic Proverb.
Note also. There is a subtlety in this week’s proverb that is not evident in any of the comparable proverbs above. The phrase ‘ina thost’ is a common idiomatic form. In this case, it literally means ‘in his silence’ with the word ‘his’ a grammatical reference to the masculine noun ‘béal’ (mouth).
It is a noun phrase that is used to convey a temporary sense. For example, if you want to say I am teacher now, but want to infer that it is a temporary state of affairs, you might say, “Tá mé i mo mhúinteoir.” (Literally I am in my teacher(ness).) If you want to convey a sense of permanence then you might say “Is mhúinteoir mé.” (I am a teacher.)
Similarly, if you wanted to infer that silence is always sweet, you could use the adjective for silent, tostach. Is binn béal tostach. Since it was not used, and since the tempory form was used, then we can infer that silence is not always golden.