"Beteiligungen" in this context is most appropriately translated as "equity interests" - "investments" would be another possibility. You should always avoid using "participation" for "Beteiligung" in the sense of equity interest (it's quite meaningless in English), although I know it's much loved by non-natives.

As far as the "Belastung" is concerned, this essentially means transferring the interests held in other entities as security/collateral. Personally, I'd prefer to avoid the term "encumbrance" for such intangible assets, and agree with Ken here that it's best reserved for tangible assets.

This is a standard provision in GnbH shareholder agreements (no "partners" in a GmbH, only shareholders or members), and sometimes in AG agreements, too. The interests referred to are, indeed, interests held in other companies. On a word-for-word basis, this gives a translation along the following lines:

a) The acquisition, disposal, assignment as security/collateral to other companies of equity interests, including silent partnership interests.

However, this is pretty clumsy, so I would suggest spelling it all out in somewhat greater detail, e.g.:

a) The acquisition and disposal of equity interests, including silent participation interests, and their assignment as security/collateral to other companies.