It is often said that the theme is key, however this may result in a gargantuan job, so minor details are easier to pull off.
- Focus. Be bold and rebrand or cull anything that clashes, and add new stuff.
- Money: give each coin a name, slang name, obverse and reverse pictures.
- Slag: make a sheet with some slag, such as loanwords from a language that isn't common.
- vulgar intensifiers
- Linguistic fillers
For example, if something is Arabian themed, add a splash of fantasy such as glass buildings (magically-supported) in the desert, rebrand elves and dwarves as djinn, swap griffins not for flying carpets, but for sand mantas as the former are too cliche, make heat exhaustion rules and so forth. This can be a minor detail or quite an undertaking (such as the aforementioned Arabian theme). For example, a game I have played with a minor detail which is simple yet drastic is playing in a small enclosed world 100 km across. However, I would strongly recommend the small detail as the focus to avoid spending ages on a campaign than may fizzle in a few weeks.
Regarding money, a coin for each metal works fine, otherwise it's just complicating stuff. A common mod that helps is rescaling as a gold piece is pittance in D&D and the denominations are useless.
Regarding slag, the vulgar intensifier is the biggest selling point: the only think worth remembering from Battlestar Galactica is the word fracking. The vulgar intensifier need not to be literal translations. For example, puitha- is Sindarin for to f**k, so if it were a loanword it be puithing, as in "This idea is puithing mad."However, it's a bit odd that elves use slang swear words and that f**king is the intensifier.
In fact, in Italian cazzo di is used (penis of) and in French putain de (prostitute of). Wiktionary has a table of some translations. If say the slang was from dwarves it would make more sense. Tolkien saw based dwarvish (khazdul) on semitic languages, so if they were to swear a Hebrew word would do: Wiki says the intensifier in Hebrew is אחושרמוטה (akhusharmúta), which google fails to tells me what it mean outside of the intensifier role and it's not that short. However due to Garry Gygax and RA Salvatore and solidified by Peter Jackson's movies dwarves are Scottish, so fecking would be a choice. However, other words might work best.
I'd recommend modifying a preëxisting less common language to avoid words cliches like Zykon. I'd pick from Klingon (harsh), Maori (syllabic) and Gaelic (melodious) —making the spelling more English— while I'd keep away from Italian or Japanese to avoid stereotypes (sleazy pizzaioli and honorable samurai). Anglish (modernized Old English) could be handy as it's obviously very English.
I think between 5–20 words works best. With a few words the game changes shape and give players instantaneous a way to speak in character that is different than out of character.