Landscape words have a sneaky way of carrying stories — and 'glen' is one of those little packages of atmosphere. To my mind, the synonym that translates best across most languages is 'valley.' It's the broad, geologic core of what a glen is: a low area between hills or mountains. Because it's a basic landform term, almost every language has a straightforward equivalent — French 'vallée,' Spanish 'valle,' German 'Tal,' Russian 'долина' (dolina), Italian 'valle,' Japanese '谷' (tani), Chinese '谷' (gǔ) or '山谷' (shāngǔ), Arabic 'وادي' (wadi), Hebrew 'עמק' (emek), Hindi 'घाटी' (ghati), Turkish 'vadi' — and so on. That makes it a reliable choice when you want your meaning to land intact across cultures and dictionaries.
That said, translation is rarely only about literal words. A glen often connotes a narrow, wooded, sometimes river-cut valley — so I always think of 'valley' as the safest umbrella term, but translators will add modifiers where needed: 'narrow valley,' 'wooded valley,' 'secluded valley,' or even borrow directly from Gaelic with 'gleann' when the Scottish character matters. Other synonyms like 'dell,' 'dale,' 'vale,' 'gorge,' or 'ravine' each carry specific size, steepness, or stylistic flavors. 'Dell' and 'dale' are charmingly English and poetic, but they don’t map neatly into many languages without sounding quaint or archaic. 'Gorge' and 'ravine' translate well where steepness is central — French 'gorge' or 'ravin,' Spanish 'barranco' — but they shift the image to something more dramatic and erosive than a gentle glen.
In casual multilingual conversation I often default to 'valley' and then paint with adjectives. If I'm writing fiction or translating poetry where mood is everything, I might keep 'glen' or use a local loanword to preserve place-specific texture: Scottish Gaelic 'gleann' or even the Welsh 'glyn' (which lives on in place names). So, for clarity and cross-linguistic reliability: pick 'valley' and tweak with modifiers when you need to capture the narrower, cozier, or wilder shades of a true glen. Personally I like when a single extra adjective — 'narrow, tree-lined valley' — gives the reader the whole scene without footnotes, and that feels like the sweet spot between precision and poetry.