In French, Wales is "Pays de Galles", so it would seem likely that Romanian got the name, or at least the word "Gal" from French, where the word "Galle" appears to be a corruption into French from Anglo-Saxon or old Germanic. From French Wikipedia:
"Le nom anglais, Wales, est issu du mot germanique Walh, qui signifie « parlant une langue celtique ou romane ». Il a donné Galles en français car le W germanique est devenu G en français (ex. : Wilhelm = Guillaume, Walho = pays gallo)"
(The English name, Wales, comes from the Germanic Walh, which means "speaking a Celtic or Roman language" (i,e. for an Anglo-Saxon, a foreigner). It came into French as "Galles" because the Germanic W became G in French, eg Wilhelm > Guillame)
Any reason not to say 'This is the country of Wales' i'm English and it seems fine to me - for example if you are showing a visitor round and reach the top of the black mountains and looking west with a sweeping gesture of your hand proclaim 'This is the country of Wales'
It's not accepted because although the literal meaning of "Țara Galilor* may be "the land of the Welsh", it's simply the standard proper name in Romanian (notice that Țara starts with a capital) for the country whose standard name in English is Wales, not the land of the Welsh, however good, even poetic, English that might be. As the song says:
"When you come home again to Wales"
- "Când vii din nou acasă în Țara Galilor"