You can supply Google translate your own glossary when asking for translations. Though not really sufficient in general, imho, it fits nicely with the way translations are handled here at translatewiki.net since we can have glossries local to projects, extensios, or even message groups, if we decide so. I have on my agenda to dig into supplying glossaries (pages in TWN) to Google Translate, but since Google translate does not do Colognian/Ripuarian, which I would want it for, it's low on my priority list.
There is a pretty versatile open souce translation tool, OmegaT, that has a glossary functionality allowing one to select from a list of likely terms. I've only briefly tested it but could not get along with the glossary functionality.