Offensive language is a common classification issue that most, if not all of the current ratings boards deal with; how each ratings board deals with words ranging from mild to strong wildly varies, which is why some similarities and differences have been provided here.
The BBFC regularly deal with offensive language as a classification issue; dividing it into categories such as 'very mild', 'mild', 'moderate', 'strong', and 'very strong' in consumer advice.
Very mild language ('hell', 'God', 'screwed') is allowed infrequently at the U category. Mild bad language ('crap', 'ass', 'bloody') is permitted at the PG category, while moderate language ('bitch', 'dick') is inconsistently considered PG and 12A/12, depending on the context/amount of times the words are used. Strong language ('f***') is allowed infrequently at 12A/12, though the BBFC is more lenient on the definition of 'infrequent' than the MPAA is; for example, the film About Time was permitted with five uses of the F-word to receive a 12A/12 rating for 'infrequent strong language, moderate sex references'.
More frequent strong language is limited to the 15 category, while very strong language ('c**t') is limited at the 15 category by number of uses and the context. The BBFC used to be much stricter on very strong language, causing any use of 'c**t' to receive an automatic 18 rating.
A notable major difference between the BBFC and other ratings system is their views on the words 'spaz' and 'spastic', which the BBFC views as being offensive in a children's work. Use of these words usually results in an automatic 12A/12 rating, and several children's works from outside the UK ave been cut for a U/PG to remove the words.
The BBFC was among the ratings boards that gave a more lenient rating to The King's Speech, rating it 12A/12 for "strong language in a speech therapy context".
There are no set limits on language at the various categories, though most provinces (with the exception of Quebec) stay close to the MPAA decision, rating films with frequent strong language with a 14A rating.
British Columbia sometimes chooses to go against these decisions depending on the context in which the offensive language is used, rating films such as Boyhood and Pirate Radio as PG despite their strong language.
Quebec has their own system unique from the rest of Canada; this system allows offensive language to be used in a G-rated work as long as it is not prominent throughout the work. Works in which it is more prominent typically receive a 13+ rating; it is very rare for a work to be classified 16+ or 18+ simply based on offensive language.
The ESRB allows for infrequent mild language to occasionally appear in an E-rated game and more frequently in an E10+ game. Moderate language is restricted to the T rating, while strong language is an automatic M rating, with only a few notable exceptions (some versions of Crazy Taxi, The Orion Conspiracy).
FSK (Germany) Edit
FSK is overall very lenient on offensive language; very rarely is it a classification issue all of its own. Movies that have been rated R in the United States for strong language such as Billy Elliot and The King's Speech have received children's classifications (FSK 6 and FSK 0 respectably).
IFCO (Ireland) Edit
IFCO's guidelines are very close to that of the BBFC, though some exceptions have occurred. For example, La La Land, which was rated 12A by the BBFC for a single use of 'f***', was rated PG in Ireland.
Kijkwijzer (Netherlands) Edit
Kijkwijzer is very lenient on offensive language; a work cannot be rated above the AL (All Ages) rating for offensive language alone.
MDA (Singapore) Edit
Some mild to moderate coarse language is allowed in PG and PG13 films; the word 'f***' is also allowed infrequently at the PG13 category. Strong language is officially allowed starting at the NC16 category, though depending on the frequency and context may be classified as M18 or R21.
Strong religious profanity (such as 'Jesus f***ing Christ') is viewed as highly offensive, and is a compulsory cut even at the R21 category.
One of the things the MPAA is most well-known for is an urban legend about the number of uses of 'f***' allowed in a PG-13 film; the MPAA has never gone on record saying there is an official limit, though there does appear to be a limit at one or two for the majority of films.
However, films such as Gunner Palace and The Hip Hop Project have managed to well exceed this limit after getting their R ratings appealed. Both films were documentaries in which the distributor argued it was important for teenagers to see the issues presented in their films.
OFLC (Australia) Edit
The OFLC has similar guidelines to the MPAA, CHVRS, and BBFC for the G and PG ratings, though on rare occasions strong language has been allowed at the PG category (ex. Julie & Julia, rated PG on appeal).
The most unique difference to the OFLC system is the M rating; there is no set limit to the number of times strong offensive language can be used, as long as the use of the words only has what the OFLC determines as a moderate impact. Similar to the BBFC, very strong language is looked at more harshly and will typically automatically place a work at the MA15+ category.
OFLC (New Zealand) Edit
The New Zealand OFLC usually has very close or identical decisions to that of the Australian OFLC, with the very rare exception. For example, Planes, Trains, & Automobiles was rated M in Australia based off one scene with strong coarse language, but only received a PG in New Zealand.
PEGI's stance on language is stricter than that of the ESRB: any use of profanity, even 'damn'/'hell', is an automatic PEGI 12 rating. The PEGI 12 rating covers mild to moderate language (this includes racial slurs), while sexual expletives are an automatic PEGI 16 rating.
Similar to the BBFC, the word 'spaz'/'spastic' is viewed much harshly than it is in other countries, and is considering an expletive that can automatically earn a game a PEGI 12 rating. (Example: Original copies of Sonic Rush Adventure which contain a single use of the word are rated 12; a reissue with the word removed received a 3 rating.)