Asturian llagu, Portuguese lago, French lac, Italian lago, Aromanian lac, Romanian lac, Sardinian lagu
Old Irish loch "lake"
Old Norse lǫgr "sea," Old High German lahha "puddle," Old Saxon lagu "lake," Old English lagu 'id.'
Old Church Slavonic lokъvi "puddle," BCS lȍkva 'id.'
Ancient Greek λάκκος (lákkos) "lake"
Basque lak(h)u "lake" (borrowed from Latin)
As mentioned above, the word lago may in fact be a remnant of a Proto-Indo-European ablaut with a root-vowel *a. This is controversial at best, and not reflected in the etymologies in our dictionary, however the bare possibility demands mentioning."Although the evidence is sparse, it appears that roots with a as fundamental vowel also ablauted. The root *sal- 'salt' had a zero-grad *sl̥-...; the root *nas- 'nose' has a lengthened-grade derivatives such as Latin nār-ēs and English nose, both from *nās-; and the root *laku- 'body of water' (Lat. lacus 'lake', Gk. lákkos 'pond') had an o-grade form *loku- that became Scottish Gaelic loch 'lake'. The view that roots in a ablauted is not universally accepted, but these forms are difficult to explain otherwise." ~ B. Fortson, Indo-European Language and Culture (2011)