1) Cover all the ways a searcher will say company: translation company, translation companies, translation firm, translation agency, etc.
2) Cover all the different ways a searcher will think about translation: translation, localization, interpreter, multilingual transcription, etc.
3) Cover all the types of translation services you offer: document translation services, website localization, escort interpreting, over the phone interpreters
4) Cover locales – countries and languages that you offer services in (if applicable): translation companies in New York, Chinese interpreters, etc.
5) Cover all the singular and plural variations of all of your words – AdWords does not do this automatically

Now, put these all together and you’ll have a nice set of keywords, for example:
translation companies in new york
trial interpreter chinese
website localization firms
web site localization firm
etc.

Some other more advanced tips to get more bang for your buck:
1) Try out phrase and exact match for your keyword terms that are shorter (3 words) and broad and phrase for those that are longer (4 words)
2) Add negative keywords for all the services that you do not offer, or languages you do not software: -software, -multimedia, -french etc. This makes sure your ads don’t appear for people who are looking for something you don’t have
3) Organize your keywords into small little groups (translation companies, website localization, phone interpreters, etc.) and write ad copy that has those exact terms in it. The matched terms will be highlighted and can get you more clicks.