Alright, it is time you mofos. I am straight going to talk about this nonsensical language shtuff that lazy writers use to make their characters names/lingos sound "foreign" to a non-Inglish ear. Because this is aimed primarily at native speakers of teh English (whether it be the Queen's or that collection of dialects known primarily as 'Murican) you foreign folks prolly only need to skim it, cause really, all you need to do to make something sound foreign to us rulers of the world (I don't want to hear anything from you Brits, we basically took over the whole primary world power thing from you, so I know that in you time you would've been slightly put out if anyone worth talking to didn't speak English) is use your native tongue.
Sin the First: Apostrophes. I mean, really, slapping an apostrophe in the middle of a word isn't just stupid, it's lazy. Unless if you're writing a Sco'ish character, there's really no reason to be using glottal stops just to sound special. Exceptions include: Scots, 'Arabs (yes, the "proper" transliteration of Arab would include an apostrophe seeing as the 'ayin is generally written with an apostrophe when transcribed into English because there's really no other way to write "I'm currently choking on a cock"); if your conlang actually has glottal stops or a voiced pharyngeal fricative (in the later case might I recommend ʕ as an approximation due to its use in IPA? No? I didn't think so... :c) feel free to use the apostrophe. Everyone else, can make ʕ noises around the gratuitous I will shove down your hatch.
Sin the Second: C/Ck/K diskishness. Where the First just reeks of laziness, the second is that of pretentiousness. Yes, there are real religions that think that Magick (with a capital M and a ck) are a thing, but I'm pretty sure that you're not writing about that in your fantasy setting. If you are, feel free to disregard and have fun with mages that can't fireball. Everyone else, STOP IT!!!!! You're not fooling anyone into thinking that you're "legit" or "have new ideas." In a fantasy setting magic == magick == magik, no ifs ands or buts, and that goes for anything else with a c that you're substituting a ck for, or a k you're subbing in a hard c for, it doesn't matter, don't do this. If you commit this sin you will be condemned to the ninth circle of hell, to warm Satan's juice crew for all of eternity (with your mouth). Do not pass go, do not collect 2,000 nuyen.
Sin the Third: Nu instead of new. I... Just... no... ... ... Please, don't... I mean, English isn't exactly the most logical, or beautiful, or perfect, or languagey language out there, but why. I may not have much respect for the language, but I'm not about to substitute a "u" for "ew" just because "it looks like a thing kids will do." If you still feel the need to do this, remember this one phrase "If I actually do this and I get famous, no kid anywhere will ever use that word ever" and you should be able to prevent yourself from butchering this poor defenseless language any more than it already has been by Shakespeare.
Sin the Fourth: *h****e. This is, combined with Sin the Second, is where we get names like Kharne (Carnage -> drop 2nd syllable -> C to K -> +*h***e = Kharne). Other words that follow this rule are therazine and theramine and roughly a gogool other words, all made up by someone that thought they were being hhote shhite, when they decided to do it but didn't realize that everyone else was doing it too. No matter how much to want to write that slashfic about your OC Phorne and his adventures with Spock, don't.
Sin the Fifth: Ps(pit it out you useless waste of air). The only reason we have "Ps" as an actual diphthong in the English language is because some highly educated idiots in the past decided out language wasn't already messed up enough by the Norman invasion, so why not throw some Greek and Latin false roots into the language, just so that eight year olds can will college scholarships for their amazing ability to spell words. DO. NOT. CONTINUE. THIS. ABUSE. OF. MY. LANGUAGE. I will personally hunt you down and shove those "Ps"'s so far past your colon you'll be tasting them when you burp, for a month. And before you say anything, no, it does not make you cool to creatively misspell words beginning in 's' solely to make your word look cooler. On a side note, I'm going to include the diphthong "ph" in this, because it really doesn't matter if you call your magical means of communication a phone or a fone, they're still the same thing (bad example, I know, but this whole sentence was written about the same time as the comment on ellipsises, so you can go phrack an asteroid.) Also "ae" when and "a" would do, and "gh" and... You know what, forget it, call them phayrighies for all I care, power to you, I hope you choke and die on all the extra letters.
Sin the Sixth: Doubling of consonants. "But Professor P," you're saying, "that fragment doesn't have a verb." My reply is to beat you with a turgid fish. At this point I'm nearing the point of incomprehensibility in my drunkeness, so you're going to have to accept my word that this is a Bad Thing™ and that you shouldn't do it. I will be honest with you and say that it's so far down the list I didn't even think of it until Tusserk decided to say something and accidentally doubled the k at the end of e's name, at which point I replied that while the use of hard vowels at the end of a name to signify your's is a warrior culture is a thing, it's not actually a crime against all languages everywhere, though it's close. In fact, compared to the others it's downright tame, mainnly because mosst peopple donn't go arounnd doubbling the midddle consonnant in their worrds. In case if you were still wondering why it's actually a bad thing, look ye to the preceding sentence.
Among those things that don't quite make it into the category of sins, but merely minor annoyances, are:::
Left to Right writing bias: Because most of our exposure to conlangs comes to us though a couple of phases transliterated into the Latin alphabet and the occasional name, this doesn't actually come up that often, but when you start seeing the actual characters in a language, how often are they presented right to left, as opposed to boustrophedon, left to right, up to down, or down to up? Those are all actual way of writing that have been (or still are) used in our world, ignoring such wonderful options as corkscrews, it doesn't matter, and "I write the words on bones and stick them in a bag, how the recipient reads them is up to the gods."
Subject, Verb, Object bias: Forgivable because most (SVO and SOV the two most common structures account for three quarters of sentencing in the world, wikipedia counted) of the languages spoken in meat space are of the SVO orientation, it is, however, not the only form of structure found in natural languages. But, I'm calling upon you oh nameless hordes of peoples making up languages to fill that empty hole in your life, make something that's VSO, or SOV, or even OSV. It can be done, the Warao people of the Guyanas did it, so I'm sure you can too.
Abuse of the Common Ellipsis: I will admit that I am a major abuser of this beautiful collection of dots. Every day I can be found using them, over and over and over... But it's all right, really. They exist to provide a visual cue for something that is lacking in a text based medium, namely the sound "um" or the physical action of rubbing your hand on the back of your head.
Umlauts (and other diacritics): If you're just using them because they look metal, don't. They have an actual meaning, and you are ruining them, you personally. If there is any higher power(s) s/he/they will take you behind the celestial outhouse and beat you with a hose if you use these beautiful symbols solely because they make Fáirïes happy. However if you're going to use them for their actual meaning, namely showing with a specificity just what particular vowel sound you're trying to use, feel free.