I don't think there is any language that actually adopts a 1:1 mapping of letters to sounds, but many come very close to adopting a 1:1 mapping between groups of letters and groups of sounds (for a particular accent).
For example, Romanian is a pretty phonetic langauge, and it adds 4 letters/diacritics to the latin alphabet to come closer - ă [uh], î (no equivalent in English), ș [sh], and ț (like [zz] in pizza).
Even so, it also has some letter combinations - ce sound like the [chai] in "chair", ci sounds like the [Chi] in "China"; but c in any other place represens a hard [k] sound. To mark a hard [k] before e or i, you then use che,chi (why they didnt do it the other way around is beyond me, since [ca],[cu],[co],[că],[cî] all spell the same [c] sound as [che], [chi]).
The same rules apply to g - ghi sounds like the [gea] in gear, while gi sounds like the [gi] in gin.
X of course represents [ks], but can also reprezent [gz] in certain words, such as examen (exam).
I is normally a particular vowel or semivowel sound, like [ee] in English. However, in word final positions after a consonant, it's usually a consonantic sound. For example, the word "mări" (seas) is pronounced as a single syllable, with just a short [ee] sound at the end. However, the word "{a} mări" ({to} enlarge) is pronounced in two syllables, [muh-ree], accenting the second syllable.
The personal pronouns I, he, and she (eu, el, ea) are all written with an e at the start, but always pronounced with an extra [ee] sound. The phonetic spelling would have been ieu, iel, iea but for whatever reason this was not done. No other words share this feature in the standard accent, though it happens to many/all words starting with the letter e- in other accents.
As with many other languages, borrowed words tend to preserve the original spelling, even after they settle on a Romanian pronunciation. For example, the word "weekend" is written exactly like this, not the Romanian phonetic spelling "uichend". Older borrows do get this treatment though. For example the French "chaise longue", a reclining chair, was borrowed into Romanian a long time ago and is now spelled "șezlong".
