Much depends on the point at which the words entered the language, and whether we think of them as loan words or not. Take the French suffix 'age', for example. The words 'village' and 'marriage', for example, were absorbed into English many centuries ago, and they took on an anglicised pronunciation : the stress shifted away from the final syllable, which turned into the anglicised /ɪdʒ/. Compare those words with the later French imports 'sabotage' and 'camouflage', which have retained their French pronunciation. These words have the same final syllable, but it is pronounced as in French: /ɑːʒ/. There are no hard and fast rules when it comes to words of foreign origin. Nor is there even a definition of what counts as 'foreign' - if you go far back enough, a very large proportion of our words come from other languages. English is a mongrel language made up of thousands of 'foreign' words. Many Spanish words in daily use are fairly recent imports into English - names of Mexican foods, styles of Latino music and so on. When we use them, we are conscious of them being Spanish words, so we at least make an effort to follow rules of Spanish pronunciation - pronouncing the 'j' in fajita as a 'h', for example. Needless to say, when a native Spanish speaker hears a native English speaker saying 'fajita', it sounds very 'English' to them - but at least we make an effort! How do we know how to pronounce these words? By repeated exposure, as you say. Remember that all learn to speak before we read or write. We learn the sounds first, and the spelling later. So a small American child will first hear their parents say the names of Mexican dishes, for example and then they just copy the sounds. Much later, the child will learn the rules of English spelling and might at some point wonder why 'tortilla' has a double 'll' in it rather than a 'y'. Or maybe they won't - after all, the English has some bizarre spellings of its own.