It's not really clear what your question is. Certainly the pronunciation of the same letter is different between different languages using the same (or nearly the same) alphabet. In some cases, the same pronunciation is indicated by different letters: Swedish and Norwegian use ö and ø, respectively, both to indicate the sound [ø].

The modern letter j is descended from the Latin letter i. Latin itself did not distinguish them. This is a somewhat similar situation to the various languages using Han characters, where while the characters are broadly speaking the same, there are some variations which carry meaning.

These differences don't suffer from the problems of Han unification within Unicode. Because the alphabets are much smaller than the set of Chinese characters, there is no problem to replicate every single small difference. This means that even letters which are clearly "the same", like the Greek letter "omicron" and the Latin letter named (in English) "oh" actually have different codepoints.

One last semi-related point: handwriting varies quite widely between different countries using the Latin alphabet, meaning that while on the internet letters may seem to be identical, they can have wildly different shapes!

é and è make different sounds in french but everybody can notice the difference

On the other side of things, in Hungarian accents are completely vertical

Both use the same font, but have the stroke on top facing different directions indicating they are from different languages.

Languages using the Latin alphabet use varying sets of diacritics, often to introduce ways to express sounds that may not have been present in (Vulgar) Latin. A particular diacritic or diacritic-letter combination can generally not be associated with any specific Latin-script-based language, of which there are many hundreds if not thousands (depending on where one draws the line between language and dialect). An interesting example is Vietnamese, a tonal language using the Latin alphabet, which uses a large number of diacritics to express tonality.

If the question is whether the Latin languages use letters differently: yes, every language is different?

I can speak for English, Spanish, and French. English is a bastard language with more exceptions than rules, as we all know. Spanish mostly uses accents as a pronunciation guide with some exceptions, whereas French accents change the letter sound more consistently. Both can change the meaning. French uses ç but not Spanish, and Spanish uses ñ exclusively. French is much more contextual.

Sp: "Si llego a tiempo" (if I arrive in time) vs "Sí, llegó a tiempo" (yes, he arrived in time). Same general sound, different emphasis.

Fr: «Bon mais sale» (good but dirty) vs «bon maïs salé» (good salted corn). Wildly different sounds and even syllable counts.

Yes different languages use different sets of characters, most of them overlapping in the case of Latin derived characters. It would be vastly less convenient if there was a completely different set (with mostly the same characters) for every single language. And what about variants over time and region and dialect? So it is much better to model them as subsets of the same larger character set. Note that À and Á are different characters with different Unicode symbols.

The human world is messy with lots of inconsistencies and irregularities, particularly from pre-computer times because humans just deal with them and tolerate mistakes. This is a challenge for modelling properly in computer systems. It does not surprise me that there are even bigger challenges around this for Kanji / Hanzi.