Though katakana is used to transcribe the pronunciation of foreign words, it usually doesn't have any relation to the way a word is written in the foreign language. It will be a phonetic transcription.

Most katakana words come from English, but there are also katakana words derived from Portuguese, German, French, Dutch, Italian and Korean (to name a few).

Single consonants:
Since English has many more sounds than Japanese a lot of the distinctive sounds in English are combined into the limited number of Japanese sounds. Japanese only knows one single consonant: the "n" (ン). Other single consonants are transcribed as follows:

Some consonants are doubled when a syllable or word ends on this consonant, while some are replaced by a long vowel.

In all of these cases (except the long vowels) the vowel sounds are pronounced as a glottal stop.

Because the "u" in katakana is often hardly pronounced, it may be necessary to add a ー to get the Japanese to actually pronounce the vowel.

Foreign words in kanji
Some foreign words have become so integrated into the Japanese language that kanji have been assigned. Also certain Chinese names maintain their kanji, though their pronunciation would normally require katakana.

The Roman alphabet in Japanese
These are the pronunciations and transcriptions of our alphabet.

Katakana and dictionaries
Most katakana words can be found in Japanese dictionaries, therefore the transcription is not up to the speaker or translator. If katakana words are pronounced or transcribed wrong, the Japanese will not understand what you are on about.

Transcription variations and abbreviations
Some katakana words are being simplified or abbreviated in Japanese. These usually take the form of removing the final chouonfugou, or taking the first two syllables of each word.

Country names +人
Adding 人 (jin) to a country name signifies an inhabitant of that country.